B. ELEMENTARY SPECIESLECTURE IIELEMENTARY SPECIES IN NATURE
B. ELEMENTARY SPECIESLECTURE IIELEMENTARY SPECIES IN NATURE
LECTURE II
ELEMENTARY SPECIES IN NATURE
What are species? Species are considered as the true units of nature by the vast majority of biologists. They have gained this high rank in our estimation principally through the influence of Linnaeus. They have supplanted the genera which were the accepted units before Linnaeus. They are now to be replaced in their turn, by smaller types, for reasons which do not rest upon comparative studies but upon direct experimental evidence.Biological studies and practical interests alike make new demands upon systematic botany. Species are not only the subject-material of herbaria and collections, but they are living entities, and their life-history and life-conditions command a gradually increasing interest. One phase of the question is to determine the easiest manner to deal with the collected forms of a country, and another feature is the problem [33] as to what groups are real units and will remain constant and unchanged through all the years of our observations.Before Linnaeus, the genera were the real units of the system. De Candolle pointed out that the old common names of plants, such as roses and clover, poplars and oaks, nearly all refer to genera. The type of the clovers is rich in color, and the shape of the flower-heads and the single flowers escape ordinary observation; but notwithstanding this, clovers are easily recognized, even if new types come to hand. White and red clovers and many other species are distinguished simply by adjectives, the generic name remaining the same for all.Tournefort, who lived in the second half of the 17th century (1656-1708), is generally considered as the author of genera in systematic botany. He adopted, what was at that time the general conception and applied it throughout the vegetable kingdom. He grouped the new and the rare and the previously overlooked forms in the same manner in which the more conspicuous plants were already arranged by universal consent. Species were distinguished by minor marks and often indicated by short descriptions, but they were considered of secondary importance.Based on the idea of a direct creation of all [34] living beings, the genera were then accepted as the created forms. They were therefore regarded as the real existing types, and it was generally surmised that species and varieties owed their origin to subsequent changes under the influence of external conditions. Even Linnaeus agreed with this view in his first treatises and in his "Philosophical Botany" he still kept to the idea that all genera had been created at once with the beginning of life.Afterwards Linnaeus changed his opinion on this important point, and adopted species as the units of the system. He declared them to be the created forms, and by this decree, at once reduced the genera to the rank of artificial groups. Linnaeus was well aware that this conception was wholly arbitrary, and that even the species are not real indivisible entities. But he simply forbade the study of lesser subdivisions. At his time he was quite justified in doing so, because the first task of the systematic botanists was the clearing up of the chaos of forms and the bringing of them into connection with their real allies.Linnaeus himself designated the subdivisions of the species as varieties, but in doing so he followed two clearly distinct principles. In some cases his species were real plants, and the varieties seemed to be derived from them by [35] some simple changes. They were subordinated to the parent-species. In other cases his species were groups of lesser forms of equal value, and it was not possible to discern which was the primary and which were the derivatives.These two methods of subdivision seem in the main, and notwithstanding their relatively imperfect application in many single examples, to correspond with two really distinct cases. The derivative varieties are distinguished from the parent-species by some single, but striking mark, and often this attribute manifests itself as the loss of some apparent quality. The loss of spines and of hairs and the loss of blue and red flower-colors are the most notorious, but in rarer cases many single peculiarities may disappear, thereby constituting a variety. This relation of varieties to the parent-species is gradually increasing in importance in the estimation of botanists, sharply contrasting with those cases, in which such dependency is not to be met with.If among the subdivisions of a species, no single one can be pointed out as playing a primary part, and the others can not be traced back to it, the relation between these lesser units is of course of another character. They are to be considered of equal importance. They are distinguished from each other by more than [36] one character, often by slight differences in nearly all their organs and qualities. Such forms have come to be designated as "elementary species." They are only varieties in a broad and vague systematic significance of the word, not in the sense accorded to this term in horticultural usage, nor in a sharper and more scientific conception.Genera and species are, at the present time, for a large part artificial, or stated more correctly, conventional groups. Every systematist is free to delimit them in a wider or in a narrower sense, according to his judgment. The greater authorities have as a rule preferred larger genera, others of late have elevated innumerable subgenera to the rank of genera. This would work no real harm, if unfortunately, the names of the plants had not to be changed each time, according to current ideas concerning genera. Quite the same inconstancy is observed with species. In the Handbook of the British Flora, Bentham and Hooker describe the forms of brambles under 5 species, while Babington in his Manual of British Botany makes 45 species out of the same material. So also in other cases. For instance, the willows which have 13 species in one and 31 species in the other of these manuals, and the hawkweeds for which the figures are 7 and 32 [37] respectively. Other authors have made still greater numbers of species in the same groups.It is very difficult to estimate systematic differences on the ground of comparative studies alone. All sorts of variability occur, and no individual or small group of specimens can really be considered as a reliable representative of the supposed type. Many original diagnoses of new species have been founded on divergent specimens and of course, the type can afterwards neither be derived from this individual, nor from the diagnosis given.This chaotic state of things has brought some botanists to the conviction that even in systematic studies only direct experimental evidence can be relied upon. This conception has induced them to test the constancy of species and varieties, and to admit as real units only such groups of individuals as prove to be uniform and constant throughout succeeding generations. The late Alexis Jordan, of Lyons in France, made extensive cultures in this direction. In doing so, he discovered that systematic species, as a rule, comprise some lesser forms, which often cannot easily be distinguished when grown in different regions, or by comparing dried material. This fact was, of course, most distasteful to the systematists of his time and even for a long period afterwards [38] they attempted to discredit it. Milde and many others have opposed these new ideas with some temporary success. Only of late has the school of Jordan received due recognition, after Thuret, de Bary, Rosen and others tested its practices and openly pronounced for them. Of late Wittrock of Sweden has joined them, making extensive experimental studies concerning the real units of some of the larger species of his country.From the evidence given by these eminent authorities, we may conclude that systematic species, as they are accepted nowadays, are as a rule compound groups. Sometimes they consist of two or three, or a few elementary types, but in other cases they comprise twenty, or fifty, or even hundreds of constant and well differentiated forms.The inner constitution of these groups is however, not at all the same in all cases. This will be seen by the description of some of the more interesting of them. The European heartsease, from which our garden-pansies have been chiefly derived, will serve as an example. The garden-pansies are a hybrid race, won by crossing theViola tricolorwith the large flowered and bright yellowV. lutea. They combine, as everyone knows, in their wide range of [39] varieties, the attributes of the latter with the peculiarities of the former species.Besides thelutea, there are some other species, nearly allied to tricolor, as for instance,cornuta,calcarata, andaltaica, which are combined with it under the head ofMelaniumas a subgenus, and which together constitute a systematic unity of undoubted value, but ranging between the common conceptions of genus and species. These forms are so nearly allied to the heartsease that they have of late been made use of in crosses, in order to widen the range of variability of garden-pansies.Viola tricoloris a common European weed. It is widely dispersed and very abundant, growing in many localities in large numbers. It is an annual and ripens its seeds freely, and if opportunity is afforded, it multiplies rapidly.Viola tricolorhas three subspecies, which have been elevated to the rank of species by some authors, and which may here be called, for brevity's sake, by their binary names. One is the typicalV. tricolor, with broad flowers, variously colored and veined with yellow, purple and white. It occurs in waste places on sandy soil. The second is calledV. arvensisor the field-pansy; it has small inconspicuous flowers, with pale-yellowish petals which are shorter than the sepals. It pollinates itself without the [40] aid of insects, and is widely dispersed in cultivated fields. The third form,V. alpestris, grows in the Alps, but is of lesser importance for our present discussion.Anywhere throughout the central part of EuropeV. tricolorandV. arvensismay be seen, each occupying its own locality. They may be considered as ranging among the most common native plants of the particular regions they inhabit. They vary in the color of the flowers, branching of the stems, in the foliage and other parts, but not to such an extent as to constitute distinct strains. They have been brought into cultivation by Jordan, Wittrock and others, but throughout Europe each of them constitutes a single type.These types must be very old and constant, fluctuating always within the same distinct and narrow limits. No slow, gradual changes can have taken place. In different countries their various habitats are as old as the historical records, and probably many centuries older. They are quite independent of one another, the distance being in numerous cases far too great for the exchange of pollen or of seeds. If slow and gradual changes were the rule, the types could not have remained so uniform throughout the whole range of these two species. They would necessarily have split up into thousands [41] and thousands of minor races, which would show their peculiar characteristics if tested by cultures in adjacent beds. This however, is not what happens. As a matter of factV. tricolorandV. arvensisare widely distributed but wholly constant types.Besides these, there occur distinct types in numerous localities. Some of them evidently have had time and opportunity to spread more or less widely and now occupy larger regions or even whole countries. Others are narrowly limited, being restricted to a single locality. Wittrock collected seeds or plants from as many localities as possible in different parts of Sweden and neighboring states and sowed them in his garden near Stockholm. He secured seeds from his plants, and grew from them a second, and in many cases a third generation in order to estimate the amount of variability. As a rule the forms introduced into his garden proved constant, notwithstanding the new and abnormal conditions under which they were propagated.First of all we may mention three perennial forms called by himViola tricolor ammotropha,V. tricolor coniophilaandV. stenochila. The typicalV. tricoloris an annual plant; sowing itself in summer and germinating soon afterwards. The young plants thrive throughout [42] the latter part of the summer and during the fall, reaching an advanced stage of development of the branched stems before winter. Early in the spring the flowers begin to open, but after the ripening of the seeds the whole plant dies.The three perennial species just mentioned develop in the same manner in the first year. During their flowering period, however, and afterwards, they produce new shoots from the lower parts of the stem. They prefer dry and sandy soils, often becoming covered with the sand that is blown on them by the winds. They are prepared for such seemingly adverse circumstances by the accumulation of food in the older stems and by the capacity of the new shoots to thrive on this food till they have become long enough to reach the light.V. tricolor ammotrophais native near Ystad in Sweden, and the other two forms on Gotland. All three have narrowly limited habitats.The typical tricolored heartsease has remained annual in all its other subspecies. It may be divided into two types in the first place,V. tricolor genuinaandV. tricolor versicolor. Both of them have a wide distribution and seem to be the prototypes from which the rarer forms must have been derived. Among these latter Wittrock describes seven local types, which [43] proved to be constant in his pedigree-cultures. Some of them have produced other forms, related to them in the way of varieties. They all have nearly the same general habit and do not exhibit any marked differences in their growth, in the structure and branching of the stems, or in the character of their foliage. Differentiating points are to be found mainly in the colors and patterns of the flowers. The veins, which radiate from the centre of the corolla are branched in some and undivided in others; in one elementary species they are wholly lacking. The purple color may be absent, leaving the flowers of a pale or a deep yellow. Or the purple may be reddish or bluish. Of the petals all five may have the purple hue on their tips, or this attribute may be limited to the two upper ones. Contrasting with this wide variability is the stability of the yellow spot in the centre, which is always present and becomes inconspicuous only, when the whole petals are of the same hue. It is a general conception that colors and color-markings are liable to great variability and do not constitute reliable standards. But the cultures of Wittrock have proved the contrary, at least in the case of the violets. No pattern, however quaint, appears changeable, if one elementary species only is considered. Hundreds of plants from seeds [44] from one locality may be grown, and all will exhibit exactly the same markings. Most of these forms are of very local occurrence. The most beautiful of all, theornatissima, is found only in Jemtland, theaurobadiaonly in Sodermanland, theanopetalain other localities in the same country, theroseolanear Stockholm, and the yellowlutescensin Finmarken.The researches of Wittrock included only a small number of elementary species, but every one who has observed the violets in the central parts of Europe must be convinced that many dozens of constant forms of the typicalViola tricolormight easily be found and isolated.We now come to the field pansy, theViola arvensis, a very common weed in the grain-fields of central Europe. I have already mentioned its small corolla, surpassed by the lobes of the calyx and its capacity of self-fertilization. It has still other curious differentiating characters; the pollen grains, which are square inV. tricolor, are five-sided inV. arvensis. Some transgressive fluctuating variability may occur in both cases through the admixture of pollen-grains. Even three-angled pollen grains are seen sometimes. Other marks are observed in the form of the anthers and the spur.There seem to be very many local subspecies [45] of the field-pansy. Jordan has described some from the vicinity of Lyons, and Wittrock others from the northern parts of Europe. They diverge from their common prototype in nearly all attributes, the flowers not showing the essential differentiating characters as in theV. tricolor. Some have their flower-stalks erect, and in others the flowers are held nearly at right angles to the stem.V. pallescensis a small, almost unbranched species with small pale flowers.V. segetalisis a stouter species with two dark blue spots on the tips of the upper petals.V. agrestisis a tall and branched, hairy form.V. nemausensisattains a height of only 10 cm., has rounded leaves and long flower-stalks. Even the seeds afford characters which may be made use of in isolating the various species.The above-mentioned elementary forms belong to the flora of southern France, and Wittrock has isolated and cultivated a number of others from the fields of Sweden. A species from Stockholm is calledViola patens;V. arvensis curtisepalaoccurs in Gotland, andV. arvensis striolatais a distinct form, which has appeared in his cultures without its true origin being ascertained.The alpine violets comprise a more widespread type with some local elementary species [46] derived exactly in the same way as the tricolored field pansies.Summarizing the general result of this description we see that the original speciesViola tricolormay be split up into larger and lesser groups of separate forms. These last prove to be constant in pedigree-cultures, and therefore are to be considered as really existent units. They are very numerous, comprising many dozens in each of the two larger subdivisions.All systematic grouping of these forms, and their combination into subspecies and species rests on the comparative study of their characters. The result of such studies must necessarily depend on principles which underlie them. According to the choice of these principles, the construction of the groups will be found to be different. Wittrock trusts in the first place to morphologic characters, and considers the development as passing from the more simple to the more complex types. On the other hand the geographic distribution may be considered as an indication of the direction of evolution, the wide-spread forms being regarded as the common parents of the minor local species.However, such considerations are only of secondary importance. It must be borne in mind that an ordinary systematic species may include [47] many dozens of elementary forms, each of which remains constant and unchanged in successive generations, even if cultivated in the same garden and under similar external conditions.Leaving the violets, we may take the vernal whitlow-grass orDraba vernafor a second illustration. This little annual cruciferous plant is common in the fields of many parts of the United States, though originally introduced from Europe. It has small basal rosettes which develop during summer and winter, and produce numerous leafless flowering stems early in the spring. It is a native of central Europe and western Asia, and may be considered as one of the most common plants, occurring anywhere in immense numbers on sandy soils. Jordan was the first to point out that it is not the same throughout its entire range. Although a hasty survey does not reveal differences, they show themselves on closer inspection. De Bary, Thuret, Rosen and many others confirmed this result, and repeated the pedigree-cultures of Jordan. Every type is constant and remains unchanged in successive generations. The anthers open in the flower-buds and pollinate the stigmas before the expansion of the flowers, thus assuring self-fertilization. Moreover, these inconspicuous little flowers are only sparingly visited by insects. Dozens of subspecies [48] may be cultivated in the same garden without any real danger of their intercrossing. They remain as pure as under perfect isolation.It is very interesting to observe the aspect of such types, when growing near each other. Hundreds of rosettes exhibit one type, and are undoubtedly similar. The alternative group is distinguishable at first sight, though the differentiating marks are often so slight as to be traceable with difficulty. Two elementary species occur in Holland, one with narrow leaves in the western provinces and one with broader foliage in the northern parts. I have cultivated them side by side, and was as much struck with the uniformity within each group, as with the contrast between the two sets.Nearly all organs show differences. The most marked are those of the leaves, which may be small or large, linear or elliptic or oblong and even rhomboidal in shape, more or less hairy with simple or with stellate branched hairs, and finally of a pure green or of a glaucous color. The petals are as a rule obcordate, but this type may be combined with others having more or less broad emarginations at the summit, and with differences in breadth which vary from almost linear types to others which touch along their margins. The pods are short and broad, or long and narrow, or varying in sundry other [49] ways. All in all there are constant differences which are so great that it has been possible to distinguish and to describe large numbers of types.Many of them have been tested as to their constancy from seed. Jordan made numerous cultures, some of which lasted ten or twelve years; Thuret has verified the assertion concerning their constancy by cultures extending over seven years in some instances; Villars and de Bary made numerous trials of shorter duration. All agree as to the main points. The local races are uniform and come true from seed; the variability of the species is not of a fluctuating, but of a polymorphous nature. A given elementary species keeps within its limits and cannot vary beyond them, but the whole group gives the impression of variability by its wide range of distinct, but nearly allied forms.The geographic distribution of these elementary species of the whitlow-grass is quite distinct from that of the violets. Here predominant species are limited to restricted localities. Most of them occupy one or more departments of France, and in Holland two of them are spread over several provinces. An important number are native in the centre of Europe, and from the vicinity of Lyons, Jordan succeeded in establishing about fifty elementary [50] species in his garden. In this region they are crowded together and not rarely two or even more quite distinct forms are observed to grow side by side on the same spot. Farther away from this center they are more widely dispersed, each holding its own in its habitat. In all, Jordan has distinguished about two hundred species ofDraba vernafrom Europe and western Asia. Subsequent authors have added new types to the already existing number from time to time.The constancy of these elementary species is directly proven by the experiments quoted above, and moreover it may be deduced from the uniformity of each type within its own domain. These are so large that most of the localities are practically isolated from one another, and must have been so for centuries. If the types were slowly changing such localities would often, though of course not always, exhibit slighter differences, and on the geographic limits of neighboring species intermediates would be found. Such however, are not on record. Hence the elementary species must be regarded as old and constant types.The question naturally arises how these groups of nearly allied forms may originally have been produced. Granting a common origin for all of them, the changes may have been [51] simultaneous or successive. According to the geographic distribution, the place of common origin must probably be sought in the southern part of central Europe, perhaps even in the vicinity of Lyons. Here we may assume that the oldDraba vernahas produced a host or a swarm of new types. Thence they must have spread over Europe, but whether in doing so they have remained constant, or whether some or many of them have repeatedly undergone specific mutations, is of course unknown.The main fact is, that such a small species asDraba vernais not at all a uniform type, but comprises over two hundred well distinguished and constant forms.It is readily granted that violets and whitlowgrasses are extreme instances of systematic variability. Such great numbers of elementary species are not often included in single species of the system. But the numbers are of secondary importance, and the fact that systematic species consist, as a rule, of more than one independent and constant subspecies, retains its almost universal validity.In some cases the systematic species are manifest groups, sharply differentiated from one another. In other instances the groups of elementary forms as they are shown by direct observation, have been adjudged by many authors [52] to be too large to constitute species. Hence the polymorphous genera, concerning the systematic subdivisions of which hardly two authors agree. Brambles and roses are widely known instances, but oaks, elms, apples, and pears,Mentha,Prunus,Vitis,Lactuca,Cucumis,Cucurbitaand numerous others are in the same condition.In some instances the existence of elementary species is so obvious, that they have been described by taxonomists as systematic varieties or even as good species. The primroses afford a widely known example. Linnaeus called themPrimula veris, and recognized three types as pertaining to this species, but Jacquin and others have elevated these subspecies to the full rank of species. They now bear the names ofPrimula elatiorwith larger,P. officinaliswith smaller flowers, andP. acaulis. In the last named the common flower-stalk is lacking and the flowers of the umbel seem to be borne in the arils of the basal leaves.In other genera such nearly allied species are more or less universally recognized.Galium Mollugohas been divided intoG. elatumwith a long and weak stem, andG. erectumwith shorter and erect stems;Cochlearia danica,anglicaandofficinalisare so nearly allied as to be hardly distinguishable.Sagina apetalaandpatula, [53]Spergula mediaandsalinaand many other pairs of allied species have differentiating characters of the same value as those of the elementary species ofDraba verna.Filago,Plantago,Carex,Ficariaand a long series of other genera afford proofs of the same close relation between smaller and larger groups of species. The European frost-weeds orHelianthemuminclude a group of species which are so closely allied, that ordinary botanical descriptions are not adequate to give any idea of their differentiating features. It is almost impossible to determine them by means of the common analytical keys. They have to be gathered from their various native localities and cultivated side by side in the garden to bring out their differences. Among the species of France, according to Jordan,Helianthemum polifolium,H. apenninum,H. pilosumandH. pulverulentumare of this character.A species of cinquefoil,Potentilla Tormentilla, which is distinguished by its quaternate flowers, occurs in Holland in two distinct types, which have proved constant in my cultural experiments. One of them has, broad petals, meeting together at the edges, and constituting rounded saucer without breaks. The other has narrow petals, which are strikingly separated from one another and show the sepals between them. [54] In the same manner bluebells vary in the size and shape of the corolla, which may be wide or narrow, bell-shaped or conical, with the tips turned downwards, sidewards or backwards.As a rule all of the more striking elementary types have been described by local botanists under distinct specific names, while they are thrown together into the larger systematic species by other authors, who study the distribution of plants over larger portions of the world. Everything depends on the point of view taken. Large floras require large species. But the study of local floras yields the best results if the many forms of the region are distinguished and described as completely as possible. And the easiest way is to give to each of them a specific name. If two or more elementary species are united in the same district, they are often treated in this way, but if each region had its own type of some given species, commonly the part is taken for the whole, and the sundry forms are described under the same name, without further distinctions.Of course these questions are all of a practical and conventional nature, but involve the different methods in which different authors deal with the same general fact. The fact is that systematic species are compound groups, exactly like the genera and that their real units [55] can only be recognized by comparative experimental studies.Though the evidence already given might be esteemed to be sufficient for our purpose, I should like to introduce a few more examples; two of them pertain to American plants.The Ipecac spurge orEuphorbia Ipecacuanhaoccurs from Connecticut to Florida, mainly near the coast, preferring dry and sandy soil. It is often found by the roadsides. According to Britton and Brown's "Illustrated Flora" it is glabrous or pubescent, with several or many stems, ascending or nearly erect; with green or red leaves, which are wonderfully variable in outline, from linear to orbicular, mostly opposite, the upper sometimes whorled, the lower often alternate. The glands of the involucres are elliptic or oblong, and even the seeds vary in shape.Such a wide range of variability evidently points to the existence of some minor types. Dr. John Harshberger has made a study of those which occur in the vicinity of Whitings in New Jersey. His types agree with the description given above. Others were gathered by him at Brown's Mills in the pinelands, New Jersey, where they grew in almost pure sand in the bright sunlight. He observed still other differentiating characters. The amount of seed [56] produced and the time of flowering were variable to a remarkable degree.Dr. Harshberger had the kindness to send me some dried specimens of the most interesting of these types. They show that the peculiarities are individual, and that each specimen has its own characters. It is very probable that a comparative experimental study will prove the existence of a large number of elementary species, differing in many points; they will probably also show differences in the amount of the active chemical substances, especially of emetine, which is usually recorded as present in about 1%, but which will undoubtedly be found in larger quantities in some, and in smaller quantities in other elementary species. In this way the close and careful distinction of the really existing units might perhaps prove of practical importance.MacFarlane has studied the beach-plum orPrunus maritima, which is abundant along the coast regions of the Eastern States from Virginia to New Brunswick. It often covers areas from two to two hundred acres in extent, sometimes to the exclusion of other plants. It is most prolific on soft drifting sand near the sea or along the shore, where it may at times be washed with ocean-spray. The fruit usually become ripe about the middle of August, and show extreme [57] variations in size, shape, color, taste, consistency and maturation period, indicating the existence of separate races or elementary species, with widely differing qualities. The earlier varieties begin to ripen from August 10 to 20, and a continuous supply can be had till September 10, while a few good varieties continue to ripen till September 20. But even late in October some other types are still found maturing their fruits.Exact studies were made of fruit and stone variations, and their characteristics as to color, weight, size, shape and consistency were fully described. Similar variations have been observed, as is well known, in the cultivated plums. Fine blue-black fruits were seen on some shrubs and purplish or yellow fruits on others. Some exhibit a firmer texture and others a more watery pulp. Even the stones show differences which are suggestive of distinct races.Recently Mr. Luther Burbank of Santa Rosa, California, has made use of the beach-plum to produce useful new varieties. He observed that it is a very hardy species, and never fails to bear, growing under the most trying conditions of dry and sandy, or of rocky and even of heavy soil. The fruits of the wild shrubs are utterly worthless for anything but preserving. [58] But by means of crossing with other species and especially with the Japanese plums, the hardy qualities of the beach-plum have been united with the size, flavor and other valuable qualities of the fruit, and a group of new plums have been produced with bright colors, ovoid and globular forms which are never flattened and have no suture. The experiments were not finished, when I visited Mr. Burbank in July, 1904, and still more startling improvements were said to have been secured.I may perhaps be allowed to avail myself of this opportunity to point out a practical side of the study of elementary species. This always appears whenever wild plants are subjected to cultivation, either in order to reproduce them as pure strains, or to cross them with other already cultivated species. The latter practice is as a rule made use of whenever a wild species is found to be in possession of some quality which is considered as desirable for the cultivated forms. In the case of the beach-plum it is the hardiness and the great abundance of fruits of the wild species which might profitably be combined with the recognized qualities of the ordinary plums. Now it is manifest, that in order to make crosses, distinct individual plants are to be chosen, and that the variability of the wild species may be of very great importance. [59] Among the range of elementary species those should be used which not only possess the desired advantages in the highest degree, but which promise the best results in other respects or their earliest attainment. The fuller our knowledge of the elementary species constituting the systematic groups, the easier and the more reliable will be the choice for the breeder. Many Californian wild flowers with bright colors seem to consist of large numbers of constant elementary forms, as for instance, the lilies, godetias, eschscholtias and others. They have been brought into cultivation many times, but the minutest distinction of their elementary forms is required to attain the highest success.In concluding, I will point out a very interesting difficulty, which in some cases impedes the clear understanding of elementary species. It is the lack of self-fertilization. It occurs in widely distant families, but has a special interest for us in two genera, which are generally known as very polymorphous groups.One of them is the hawkweed orHieracium, and the other is the dandelion orTaraxacum officinale. Hawkweeds are known as a genus in which the delimitation of the species is almost impossible, Thousands of forms may be cultivated side by side in botanical gardens, exhibiting [60] slight but undoubted differentiating features, and reproduce themselves truly by seed. Descriptions were formerly difficult and so complicated that the ablest writers on this genus, Fries and Nageli are said not to have been able to recognize the separate species by the descriptions given by each other. Are these types to be considered as elementary species, or only as individual differences? The decision of course, would depend upon their behavior in cultures. Such tests have been made by various experimenters. In the dandelion the bracts of the involucre give the best characters. The inner ones may be linear or linear-lanceolate, with or without appendages below the tip; the outer ones may be similar and only shorter, or noticeably larger, erect, spreading or even reflexed, and the color of the involucre may be a pure green or glaucous; the leaves may be nearly entire or pinnatifid, or sinuate-dentate, or very deeply runcinate-pinnatifid, or even pinnately divided, the whole plant being more or less glabrous.Raunkiaer, who has studied experimentally a dozen types from Denmark, found them constant, but observed that some of them have no pollen at all, while in others the pollen, though present, is impotent. It does not germinate on the stigma, cannot produce the ordinary tube, [61] and hence has no fertilizing power. But the young ovaries do not need such fertilization. They are sufficient unto themselves. One may cut off all the flowers of a head before the opening of the anthers, and leave the ovaries untouched, and the head will ripen its seeds quite as well. The same thing occurs in the hawkweeds. Here, therefore, we have no fertilization and the extensive widening of the variability, which generally accompanies this process is, of course, wanting. Only partial or vegetative variability is present. Unfertilized eggs when developing into embryos are equivalent to buds, separated from the parent-plant and planted for themselves. They repeat both the specific and the individual characters of the parent. In the case of the hawkweed and the dandelion there is at present no means of distinguishing between these two contrasting causes of variability. But like the garden varieties which are always propagated in the vegetative way, their constancy and uniformity are only apparent and afford no real indication of hereditary qualities.In addition to these and other exceptional cases, seed-cultures are henceforth to be considered as the sole means of recognizing the really existing systematic units of nature. All other groups, including systematic species and [62] genera, are equally artificial or conventional. In other words we may state "that current misconceptions as to the extreme range of fluctuating variability of many native species have generally arisen from a failure to recognize the composite nature of the forms in question," as has been demonstrated by MacDougal in the case of the common evening-primrose,Oenothera biennis. "It is evident that to study the behavior of the characters of plants we must have them in their simplest combinations; to investigate the origin and movements of species we must deal with them singly and uncomplicated."
[63]
LECTURE IIIELEMENTARY SPECIES OF CULTIVATED PLANTS
LECTURE IIIELEMENTARY SPECIES OF CULTIVATED PLANTS
ELEMENTARY SPECIES OF CULTIVATED PLANTS
Recalling the results of the last lecture, we see that the species of the systematists are not in reality units, though in the ordinary course of floristic studies they may, as a rule, seem to be so. In some cases representatives of the same species from different countries or regions, when compared with one another do not exactly agree. Many species of ferns afford instances of this rule, and Lindley and other great systematists have frequently been puzzled by the wide range of differences between the individuals of a single species.In other cases the differing forms are observed to grow near each other, sometimes in neighboring provinces, sometimes in the same locality, growing and flowering in mixtures of two or three or even more elementary types. The violets exhibit widespread ancient types, from which the local species may be taken to have arisen. The common ancestors of the Whitlow-grasses are probably not to be found [64] among existing forms, but numerous types are crowded together in the southern part of central Europe and more thinly scattered elsewhere, even as far as western Asia. There can be little doubt that their common origin is to be sought in the center of their geographic distribution.Numerous other cases exhibit smaller numbers of elementary units within a systematic species; in fact purely uniform species seem to be relatively rare. But with small numbers there are of course no indications to be expected concerning their common origin or the starting point of their distribution.It is manifest that these experiences with wild species must find a parallel among cultivated plants. Of course cultivated plants were originally wild and must have come under the general law. Hence we may conclude that when first observed and taken up by man, they must already have consisted of sundry elementary subspecies. And we may confidently assert that some must have been rich and others poor in such types.Granting this state of things as the only probable one, we can easily imagine what must have been the consequences. If a wild species had been taken into cultivation only once, the cultivated form would have been a single elementary [65] type. But it is not very likely that such partiality would occur often. The conception that different tribes at different times and in distant countries would have used the wild plants of their native regions seems far more natural than that all should have obtained plants for cultivation from the same source or locality. If this theory may be relied upon, the origin of many of the more widely cultivated agricultural plants must have been multiple, and the number of the original elementary species of the cultivated types must have been so much the larger, the more widely distributed and variable the plants under consideration were before the first period of cultivation.Further it would seem only natural to explain the wide variability of many of our larger agricultural and horticultural stocks by such an incipient multiformity of the species themselves. Through commercial intercourse the various types might have become mixed so as to make it quite impossible to point out the native localities for each of them.Unfortunately historical evidence on this point is almost wholly lacking. The differences in question could not have been appreciated at that remote period, and interest the common observer but little even today. The history of most of the cultivated plants is very obscure, [66] and even the most skillful historians, by sifting the evidence afforded by the older writers, and that obtained by comparative linguistic investigations have been able to do little more than frame the most general outline of the cultural history of the most common and most widely used plants.Some authors assume that cultivation itself might have been the principal cause of variability, but it is not proved, nor even probable, that cultivated plants are intrinsically more variable than their wild prototypes. Appearances in this case are very deceptive. Of course widely distributed plants are as a rule richer in subspecies than forms with limited distribution, and the former must have had a better chance to be taken into cultivation than the latter. In many cases, especially with the more recent cultivated species, man has deliberately chosen variable forms, because of their greater promise. Thirdly, wide variability is the most efficient means of acclimatization, and only species with many elementary units would have offered the adequate material for introduction into new countries.From this discussion it would seem that it is more reasonable to assert that variability is one of the causes of the success of cultivation, than to assume that cultivation is a cause of variability [67] at large. And this assumption would be equally sufficient to explain the existing conditions among cultivated plants.Of course I do not pretend to say that cultivated plants should be expected to be less variable than in the wild state, or that swarms of elementary species might not be produced during cultivation quite as well as before. However the chance of such an event, as is easily seen, cannot be very great, and we shall have to be content with a few examples of which the coconut is a notable one.Leaving this general discussion of the subject, we may take up the example of the beets. The sugar-beet is only one type from among a horde of others, and though the origin of all the single types is not historically known, the plant is frequently found in the wild state even at the present time, and the native types may be compared with the corresponding cultivated varieties.The cultivation of beets for sugar is not of very ancient date. The Romans knew the beets and used them as vegetables, both the roots and the leaves. They distinguished a variety with white and one with red flesh, but whether they cultivated them, or only collected them from where they grew spontaneously, appears to be unknown.[68] Beets are even now found in large quantities along the shores of Italy. They prefer the vicinity of the sea, as do so many other members of the beet family, and are not limited to Italy, but are found growing elsewhere on the littoral of the Mediterranean, in the Canary Islands and through Persia and Babylonia to India. In most of their native localities they occur in great abundance.The color of the foliage and the size of the roots are extremely variable. Some have red leafstalks and veins, others a uniform red or green foliage, some have red or white or yellow roots, or exhibit alternating rings of a red and of a white tinge on cut surfaces. It seems only natural to consider the white and the red, and even the variegated types as distinct varieties, which in nature do not transgress their limits nor change into one another. In a subsequent lecture I will show that this at least is the rule with the corresponding color-varieties in other genera.The fleshiness or pulpiness of the roots is still more variable. Some are as thick as the arm and edible, others are not thicker than a finger and of a woody composition, and the structure of this woody variety is very interesting. The sugar-beet consists, as is generally known, of concentric layers of sugar-tissue and of vascular [69] strands; the larger the first and the smaller the latter, the greater is, as a rule, the average amount of sugar of the race. Through the kindness of the late Mr. Rimpau, a well known German breeder of sugar-beet varieties, I obtained specimens from seed of a native wild locality near Bukharest. The plants produced quite woody roots, showing almost no sugar tissue at all. Woody layers of strongly developed fibrovascular strands were seen to be separated one from another only by very thin layers of parenchymatous cells. Even the number of layers is variable; it was observed to be five in my plants; but in larger roots double this number and even more may easily be met with.Some authors have distinguished specific types among these wild forms. While the cultivated beets are collected under the head ofBeta vulgaris, separate types with more or less woody roots have been described asBeta maritimaandBeta patula. These show differences in the habit of the stems and the foliage. Some have a strong tendency to become annual, others to become biennial. The first of course do not store a large quantity of food in their roots, and remain thin, even at the time of flowering. The biennial types occur in all sizes of roots. In the annuals the stems may vary from [70] erect to ascending, and the namepatulaindicates stems which are densely branching from the base with widely spreading branches throughout. Mr. Em. von Proskowetz of Kwassitz, Austria, kindly sent me seeds of thisBeta patula, the variability of which was so great in my cultures as to range from nearly typical sugar-beets to the thin woody type of Bukharest.Broad and narrow leaves are considered to be differentiating marks betweenBeta vulgarisandBeta patula, but even here a wide range of forms seem to occur.Rimpau, Proskowetz, Schindler and others have made cultures of beets from wild localities in order to discover a hypothetical common ancestor of all the present cultivated types. These researches point to theB. patulaas the probable ancestor, but of course they were not made to decide the question as to whether the origination of the several now existing types had taken place before or during culture. From a general point of view the variability of the wild species is parallel to that of the cultivated forms to such a degree as to suggest the multiple origin of the former. But a close investigation of this highly important problem has still to be made.The varieties of the cultivated beets are commonly [71] included in four subspecies. The two smallest are the salad-beets and the ornamental forms, the first being used as food, and ordinarily cultivated in red varieties, the second being used as ornamental plants during the fall, when they fill the beds left empty by summer flowers, with a bright foliage that is exceedingly rich in form and color. Of the remaining subspecies, one comprises the numerous sorts cultivated as forage-crops and the other the true sugar-beets. Both of them vary widely as to the shape and the size of the roots, the quality of the tissue, the foliage and other characteristics.Some of these forms, no doubt, have originated during culture. Most of them have been improved by selection, and no beet found in the wild state ever rivals any cultivated variety. But the improvement chiefly affects the size, the amount of sugar and nutrient substances and some other qualities which recur in most of the varieties. The varietal attributes themselves however, are more or less of a specific nature, and have no relation to the real industrial value of the race. The short-rooted and the horn-shaped varieties might best be cited as examples.The assertion that the sundry varieties of forage-beets are not the result of artificial selection, [72] is supported in a large measure by the historic fact that the most of them are far older than the method of conscious selection of plants itself. This method is due to Louis Vilmorin and dates from the middle of the last century. But in the sixteenth century most of our present varieties of beets were already in cultivation. Caspar Bauhin gives a list of the beets of his time and it is not difficult to recognize in it a large series of subspecies and varieties and even of special forms, which are still cultivated. A more complete list was published towards the close of the same century by Olivier de Serres in his world-renowned "Theatre d'Agriculture" (Paris, 1600).The red forage-beets which are now cultivated on so large a scale, had been introduced from Italy into France only a short time before.From this historic evidence, the period during which the beets were cultivated from the time of the Romans or perhaps much later, up to the time of Bauhin and De Serres, would seem far too short for the production by the unguided selection of man of all the now existing types. On the other hand, the parallelism between the characters of some wild and some cultivated varieties goes to make it very probable that other varieties have been found in the same way, some in this country and others in that, [73] and have been taken into cultivation separately. Afterwards of course all must have been improved in the direction required by the needs of man.Quite the same conclusion is afforded by apples. The facts are to some extent of another character, and the rule of the derivation of the present cultivated varieties from original wild forms can be illustrated in this case in a more direct way. Of course we must limit ourselves to the varieties of pure ancestry and leave aside all those which are of hybrid or presumably hybrid origin.Before considering their present state of culture, something must be, said about the earlier history and the wild state of the apples.The apple-tree is a common shrub in woods throughout all parts of Europe, with the only exception of the extreme north. Its distribution extends to Anatolia, the Caucasus and Ghilan in Persia. It is found in nearly all forests of any extent and often in relatively large numbers of individuals. It exhibits varietal characters, which have led to the recognition of several spontaneous forms, especially in France and in Germany.The differentiating qualities relate to the shape and indumentum of the leaves. Nothing is known botanically as to differences between [74] the fruits of these varieties, but as a matter of fact the wild apples of different countries are not at all the same.Alphonse De Candolle, who made a profound study of the probable origin of most of our cultivated plants, comes to the conclusion that the apple tree must have had this wide distribution in prehistoric times, and that its cultivation began in ancient times everywhere.This very important conclusion by so high an authority throws considerable light on the relation between cultivated and wild varieties at large. If the historic facts go to prove a multiple origin for the cultivation of some of the more important useful plants, the probability that different varieties or elementary species have been the starting points for different lines of culture, evidently becomes stronger.Unfortunately, this historic evidence is scanty. The most interesting facts are those concerning the use of apples by the Romans and by their contemporaries of the Swiss and middle European lake-dwellings. Oswald Heer has collected large numbers of the relics of this prehistoric period. Apples were found in large quantities, ordinarily cut into halves and with the signs of having been dried. Heer distinguished two varieties, one with large and one with small fruits. The first about 3 and [75] the other about 1.5-2 cm. in diameter. Both are therefore very small compared with our present ordinary varieties, but of the same general size as the wild forms of the present day. Like these, they must have been of a more woody and less fleshy tissue. They would scarcely have been tasteful to us, but in ancient times no better varieties were known and therefore no comparison was possible.There is no evidence concerning the question, as to whether during the periods mentioned apples were cultivated or only collected in the wild state. The very large numbers which are found, have induced some writers to believe in their culture, but then there is no reason why they should not have been collected in quantity from wild shrubs. The main fact is that the apple was not a uniform species in prehistoric times but showed even then at least some amount of variability.At the present day the wild apples are very rich in elementary species. Those of Versailles are not the same as those of Belgium, and still others are growing in England and in Germany. The botanical differences derived from the blossoms and the leaves are slight, but the flavor, size and shape of the fruits diverge widely. Two opinions have been advanced to explain this high degree of variability, but [76] neither of them conveys a real explanation; their aim is chiefly to support different views as to the causes of variability, and the origin of elementary species at large.One opinion, advocated by De Candolle, Darwin and others, claims that the varieties owe their origin to the direct influence of cultivation, and that the corresponding forms found in the wild state, are not at all original, but have escaped from cultivation and apparently become wild. Of course this possibility cannot be denied, at least in any single instance, but it seems too sweeping an assertion to make for the whole range of observed forms.The alternative theory is that of van Mons, the Belgian originator of commercial varieties of apples, who has published his experiments in a large work called "Arbres fruitiers ou Pomonomie belge." Most of the more remarkable apples of the first half of the last century were produced by van Mons, but his greatest merit is not the direct production of a number of good varieties, but the foundation of the method, by which new varieties may be obtained and improved.According to van Mons, the production of a new variety consists chiefly of two parts. The first is the discovery of a subspecies with new desirable qualities. The second is the transformation [77] of the original small and woody apple into a large, fleshy and palatable variety. Subspecies, or what we now call elementary species were not produced by man; nature alone creates new forms, as van Mons has it. He examined with great care the wild apples of his country, and especially those of the Ardennes, and found among them a number of species with different flavors. For the flavor is the one great point, which must be found ready in nature and which may be improved, but can never be created by artificial selection. The numerous differences in flavor are quite original; all of them may be found in the wild state and most of them even in so limited a region as the Ardennes Mountains. Of course van Mons preferred not to start from the wild types themselves, when the same flavor could be met with in some cultivated variety. His general method was, to search for a new flavor and to try to bring the bearer of it up to the desired standard of size and edibility.The latter improvement, though it always makes the impression of an achievement, is only the last stone to be added to the building up of the commercial value of the variety. Without it, the best flavored apple remains a crab; with it, it becomes a conquest. According to the method of van Mons it may be reached within [78] two or three generations, and a man's life is wholly sufficient to produce in this way many new types of the very best sorts, as van Mons himself has done. It is done in the usual way, sowing on a large scale and selecting the best, which are in their turn brought to an early maturation of their fruit by grafting, because thereby the life from seed to seed may be reduced to a few years.Form, taste, color, flavor and other valuable marks of new varieties are the products of nature, says van Mons, only texture, fleshiness and size are added by man. And this is done in each new variety by the same method and according to the same laws. The richness of the cultivated apples of the present day was already present in the large range of original wild elementary species, though unobserved and requiring improvement.An interesting proof of this principle is afforded by the experience of Mr. Peter M. Gideon, as related by Bailey. Gideon sowed large quantities of apple-seeds, and one seed produced a new and valuable variety called by him the "Wealthy" apple. He first planted a bushel of apple-seeds, and then every year, for nine years, planted enough seeds to produce a thousand trees. At the end of ten years all seedlings had perished except one hardy seedling [79] crab. This experiment was made in Minnesota, and failed wholly. Then he bought a small lot of seeds of apples and crab-apples in Maine and from these the "Wealthy" came. There were only about fifty seeds in the lot of crab-apple seed which produced the "Wealthy," but before this variety was obtained, more than a bushel of seed had been sown. Chance afforded a species with an unknown taste; but the growing of many thousands of seedlings of known varieties was not the best means to get something really new.Pears are more difficult to improve than apples. They often require six or more generations to be brought from the wild woody state to the ordinary edible condition. But the varieties each seem to have a separate origin, as with apples, and the wide range of form and of taste must have been present in the wild state, long before cultivation. Only recently has the improvement of cherries, plums, currants and gooseberries been undertaken with success by Mr. Burbank, and the difference between the wild and cultivated forms has hitherto been very small. All indications point to the existence, before the era of cultivation, of larger or smaller numbers of elementary species.The same holds good with many of the larger forage crops and other plants of great industrial [80] value. Clover exhibits many varieties, which have been cultivated indiscriminately, and often in motley mixtures. The flower heads may be red or white, large or small, cylindric or rounded, the leaves are broader or narrower, with or without white spots of a curious pattern. They may be more or less hairy and so forth. Even the seeds exhibit differences in size, shape or color, and of late Martinet has shown, that by the simple means of picking out seeds of the same pattern, pure strains of clover may be obtained, which are of varying cultural value. In this way the best subspecies or varieties may be sought out for separate cultivation. Even the white spots on the leaflets have proved to be constant characters corresponding with noticeable differences in yield.Flax is another instance. It was already cultivated, or at least made use of during the period of the lake-dwellers, but at that time it was a species referred to asLinum angustifolium, and not theLinum usitatissimum, which is our present day flax. There are now many subspecies, elementary species, and varieties under cultivation. The oldest of them is known as the "springing flax," in opposition to the ordinary "threshing flax." It has capsules which open of themselves, in order to disseminate the seeds, while the ordinary heads of the [81] flax remain closed until the seeds are liberated by threshing. It seems probable that the first form orLinum crepitansmight thrive in the wild state as well as any other plant, while in the common species those qualities are lacking which are required for a normal dissemination of the seeds. White or blue flowers, high or dwarf stems, more or less branching at the base and sundry other qualities distinguish the varieties, aside from the special industrial difference of the fibres. Even the life-history varies from annual and biennial, to perennial.It would take us too long to consider other instances. It is well known that corn, though considered as a single botanical species, is represented by different subspecies and varieties in nearly every region in which it is grown. Of course its history is unknown and it is impossible to decide whether all the tall and dwarf forms, or starchy and sweet varieties, dented or rounded kernels, and hundreds of others are older than culture or have come into existence during historic times, or as some assume, through the agency of man. But our main point now is not the origin, but only the existence of constant and sharply differentiated forms within botanical species. Nearly every cultivated plant affords instances of such diversity. Some include a few types only, while [82] others show, a large number of forms clearly separated to a greater or lesser degree.In some few instances it is obvious that this variability is of later date than culture. The most conspicuous case is that of the coconut. This valuable palm is found on nearly all tropical coasts, in America, as well as in Asia, but in Africa and Australia there are many hundreds of miles of shore line, where it is not found. Its importance is not at all the same everywhere. On the shores and islands of the Indian Ocean and the Malay Archipelago, man is chiefly dependent upon it, but in America it is only of subordinate usefulness.In connection with these facts, it abounds in subspecies and varieties in the East Indian regions, but on the continent of America little attention has as yet been given to its diverging qualities. In the Malayan region it affords nearly all that is required by the inhabitants. The value of its fruit as food, and the delicious beverage which it yields, are well known. The fibrous rind is not less useful; it is manufactured into a kind of cordage, mats and floor-cloths. An excellent oil is obtained from the kernel by compression. The hard covering of the stem is converted into drums and used in the construction of huts; the lower part is so hard as to take on a beautiful polish [83] when it resembles agate. Finally the unexpanded terminal bud is a delicate article of food. Many other uses could be mentioned, but these may suffice to indicate how closely the life of the inhabitants is bound up with the culture of this palm, and how sharply, in consequence, its qualities must have been watched by early man. Any divergence from the ordinary type must have been noted; those which were injurious must have been rejected, but the useful ones must have been appreciated and propagated. In a word any degree of variability afforded by nature must have been noticed and cultivated.More than fifty different sorts of the coconut are described from the Indian shores and islands, with distinct local and botanical names. Miquel, who was one of the best systematists of tropical plants, of the last century, described a large number of them, and since, more have been added. Nearly all useful qualities vary in a higher or lesser degree in the different varieties. The fibrous strands of the rind of the nut are developed in some forms to such a length and strength as to yield the industrial product known as the coir-fibre. Only three of them are mentioned by Miquel that have this quality, theCocos nucifera rutila,cupuliformisandstupposa. Among them therutila[84] yields the best and most supple fibres, while those of thestupposaare stiff and almost unbending.The varieties also differ greatly in size, color, shape and quality, and the trees have also peculiar characteristics. One variety exhibits leaves which are nearly entire, the divisions being only imperfectly separated, as often occurs in the very first leaves of the seedlings of other varieties. The flavor of the flesh, oil and milk likewise yield many good varietal marks.In short, the coconut-palm comes under the general rule, that botanical species are built up of a number of sharply distinguishable types, which prove their constancy and relative independence by their wide distribution in culture. In systematic works all these forms are called varieties, and a closer investigation of their real systematic value has not yet been made. But the question as to the origin of the varieties and of the coconut itself has engrossed the attention of many botanists, among whom are De Candolle in the middle of the last century, and Cook at its close.Both questions are closely connected. De Candolle claimed an Asiatic origin for the whole species, while Cook's studies go to prove that its original habitat is to be sought in the northern countries of South America. Numerous [85] varieties are growing in Asia and have as yet not been observed to occur in America, where the coconut is only of subordinate importance, being one of many useful plants, and not the only one relied upon by the natives for their subsistence. If therefore, De Candolle's opinion is the right one, the question as to whether the varieties are older or younger than the cultivated forms of the species, must always remain obscure. But if the proofs of an American origin should be forthcoming, the possibility, and even the probability that the varieties are of later date than the beginning of their culture, and have originated while in this condition must at once be granted. An important point in the controversy is the manner in which the coconuts were disseminated from shore to shore, from island to island. De Candolle, Darwin and most of the European writers claim that the dispersal was by natural agencies, such as ocean-currents. They point out that the fibrous rind or husk would keep the fruits afloat, and uninjured, for many days or even many weeks, while being carried from one country to another in a manner that would explain their geographic distribution. But the probability of the nuts being thrown upon the strand, and far enough from the shore to find suitable conditions for their germination, is a very small one. To insure [86] healthy and vigorous seedlings the nuts must be fully ripe, after which planting cannot be safely delayed for more than a few weeks. If kept too moist the nuts rot. If once on the shore, and allowed to lie in the sun, they become overheated and are thereby destroyed; if thrown in the shade of other shrubs and trees, the seedlings do not find the required conditions for a vigorous growth.Some authors have taken the fibrous rind to be especially adapted to transport by sea, but if this were so, this would argue that water is the normal or at least the very frequent medium of dissemination, which of course it is not. We may, claim with quite as much right that the thick husk is necessary to enable the heavy fruit to drop from tall trees with safety. But even for this purpose the protection is not sufficient, as the nuts often suffer from falling to such a degree as to be badly injured as to their germinating qualities. It is well known that nuts, which are destined for propagation, are as a rule not allowed to fall off, but are taken from the trees with great care.Summing up his arguments, Cook concludes that there is little in the way of known facts to support the poetic theory of the coconut palm dropping its fruits into the sea to float away to barren islands and prepare them for [87] human habitation. Shipwrecks might furnish a successful method of launching viable coconuts, and such have no doubt sometimes contributed to their distribution. But this assumption implies a dissemination of the nuts by man, and if this principal fact is granted, it is far more natural to believe in a conscious intelligent dissemination.The coconut is a cultivated tree. It may be met with in some spots distant from human dwellings, but whenever such cases have been subjected to a closer scrutiny, it appears that evidently, or at least probably, huts had formerly existed in their neighborhood, but having been destroyed by some accident, had left the palm trees uninjured. Even in South America, where it may be found in forests at great distances from the sea-shore, it is not at all certain that true native localities occur, and it seems to be quite lost in its natural condition.Granting the cultivated state of the palms as the only really important one, and considering the impossibility or at least great improbability of its dissemination by natural means, the distribution by man himself, according to his wants, assumes the rank of an hypothesis fully adequate to the explanation of all the facts concerning the life-history of the tree.We now have to inquire into the main question, [88] whether it is probable that the coconut is of American or of Asiatic origin, leaving aside the historic evidence which goes to prove that nothing is known about the period in which its dissemination from one hemisphere to another took place, we will now consider only the botanic and geographic evidence, brought forward by Cook. He states that the whole family of coconut-palms, consisting of about 20 genera and 200 species, are all strictly American with the exception of the rather aberrant African oilpalm, which has, however, an American relative referred to the same genus. The coconut is the sole representative of this group which is connected with Asia and the Malayan region, but there is no manifest reason why other members of the same group could not have established themselves there, and maintained an existence under conditions, which are not at all unfavorable to them. The only obvious reason is the assumption already made, that the distribution was brought about by man, and thus only affected the species, chosen by him for cultivation. That the coconut cannot have been imported from Asia into America seems to be the most obvious conclusion from the arguments given. It should be briefly noted, that it was known and widely distributed in tropical America at the time of the discovery of that continent [89] by Columbus, according to accounts of Oviedo and other contemporary Spanish writers.Concluding we may state that according to the whole evidence as it has been discussed by De Candolle and especially by Cook, the coconut-palm is of American origin and has been distributed as a cultivated tree by man through the whole of its wide range. This must have happened in a prehistoric era, thus affording time enough for the subsequent development of the fifty and more known varieties. But the possibility that at least some of them have originated before culture and have been deliberately chosen by man for distribution, of course remains unsettled.Coconuts are not very well adapted for natural dispersal on land, and this would rather induce us to suppose an origin within the period of cultivation for the whole group. There are a large number of cultivated varieties of different species which by some peculiarity do not seem adapted for the conditions of life in the wild state. These last have often been used to prove the origin of varietal forms during culture. One of the oldest instances is the variety or rather subspecies of the opium-poppy, which lacks the ability to burst open its capsules. The seeds, which are thrown out by the wind, in the common forms, through the apertures underneath [90] the stigma, remain enclosed. This is manifestly a very useful adaptation for a cultivated plant, as by this means no seeds are lost. It would be quite a disadvantage for a wild species, and is therefore claimed to have been connected from the beginning with the cultivated form.The large kernels of corn and grain, of beans and peas, and even of the lupines were considered by Darwin and others to be unable to cope with natural conditions of life. Many valuable fruits are quite sterile, or produce extremely few seeds. This is notoriously the case with some of the best pears and grapes, with the pine-apples, bananas, bread-fruits, pomegranate and some members of the orange tribe. It is open to discussion as to what may be the immediate cause of this sterility, but it is quite evident, that all such sterile varieties must have originated in a cultivated condition. Otherwise they would surely have been lost.In horticulture and agriculture the fact that new varieties arise from time to time is beyond all doubt, and it is not this question with which we are now concerned. Our arguments were only intended to prove that cultivated species, as a rule, are derived from wild species, which obey the laws discussed in a previous lecture. The botanic units are compound entities, and [91] the real systematic units in elementary species play the same part as in ordinary wild species. The inference that the origin of the cultivated plants is multiple, in most cases, and that more than one, often many separate elementary forms of the same species must originally have been taken into cultivation, throws much light upon many highly important problems of cultivation and selection. This aspect of the question will therefore be the subject of the next lecture.
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