LECTURE XXIMUTATIONS IN HORTICULTURE
LECTURE XXIMUTATIONS IN HORTICULTURE
MUTATIONS IN HORTICULTURE
It is well known that Darwin based his theory of natural selection to a large extent upon the experience of breeders. Natural and artificial selection exhibit the same general features, yet it was impossible in Darwin's time to make a critical and comparative analysis of the two processes.In accordance with our present conception there is selection of species and selection within the species. The struggle for life determines which of a group of elementary species shall survive and which shall disappear. In agricultural practice the corresponding process is usually designated by the name of variety-testing. Within the species, or within the variety, the sieve of natural selection is constantly eliminating poor specimens and preserving those that are best adapted to live under the given conditions. Some amelioration and some local races are the result, but this does not appear to be of much importance. On the contrary, the selection [605] within the race holds a prominent place in agriculture, where it is known by the imposing term, race-breeding.Experience and methods in horticulture differ from those in agriculture in many points. Garden varieties have been tested and separated for a long time, but neither vegetables nor flowers are known to exhibit such motley groups of types as may be seen in large forage crops.New varieties which appear from time to time may be ornamental or otherwise in flowers, and more or less profitable than their parents in vegetables and fruits. In either case the difference is usually striking, or if not, its culture would be unprofitable.The recognition of useful new varieties being thus made easy, the whole attention of the breeder is reduced to isolating the seeds of the mutants that are to be saved and sown separately, and this process must be repeated during a few years, in order to produce the quantity of seed that is needed for a profitable introduction of the variety into commerce. In proportion to the abundance of the harvest of each year this period is shorter for some and longer for other species.Isolation in practice is not so simple nor so easy an affair as it is in the experimental garden. Hence we have constant and nearly unavoidable [606] cross-fertilizations with the parent form or with neighboring varieties, and consequent impurity of the new strain. This impurity we have called vicinism, and in a previous lecture have shown its effects upon the horticultural races on one hand, and on the other, on the scientific value that can be ascribed to the experience of the breeder. We have established the general rule that stability is seldom met with, but that the observed instability is always open to the objection of being the result of vicinism. Often this last agency is its sole cause; or it may be complicated with other factors without our being able to discern them.Though our assertion that the practice of the horticulturist in producing new varieties is limited to isolation, whenever chance affords them, is theoretically valid, it is not always so. We may discern between the two chief groups of varieties. The retrograde varieties are constant, the individuals not differing more from one another than those of any ordinary species. The highly variable varieties play an important part in horticulture. Double flowers, striped flowers, variegated leaves and some others yield the most striking instances. Such forms have been included in previous lectures among the ever-sporting varieties, because their peculiar characters oscillate between two extremes, viz: [607] the new one of the variety and the corresponding character of the original species.In such cases isolation is usually accompanied by selection: rarely has the first of a double, striped or variegated race well filled or richly striped flowers or highly spotted leaves. Usually minor degrees of the anomaly are seen first, and the breeder expects the novelty to develop its features more completely and more beautifully in subsequent generations. Some varieties need selection only in the beginning, in others the most perfect specimens must be chosen every year as seed-bearers. For striped flowers, it has been prescribed by Vilmorin, that seeds should be taken only from those with the smallest stripes, because there is always reversion. Mixed seed or seed from medium types would soon yield plants with too broad stripes, and therefore less diversified flowers.In horticulture, new varieties, both retrograde and ever-sporting, are known to occur almost yearly. Nevertheless, not every novelty of the gardener is to be considered as a mutation in the scientific sense of the word. First of all, the novelties of perennial and woody species are to be excluded. Any extreme case of fluctuating variability may be preserved and multiplied in the vegetative way. Such types are designated [608] in horticulture as varieties, though obviously they are of quite another nature than the varieties reproduced by seed. Secondly, a large number, no doubt the greater number of novelties, are of hybrid origin. Here we may discern two cases. Hybrids may be produced by the crossing of old types, either of two old cultivated forms or newly introduced species, or ordinarily between an old and an introduced variety. Such novelties are excluded from our present discussion. Secondly, hybrids may be produced between a true, new mutation and some of the already existing varieties of the same species. Examples of this obvious and usual practice will be given further on, but it must be pointed out now that by such crosses a single mutation may produce as many novelties as there are available varieties of the same species.Summarizing these introductory remarks we must lay stress on the fact that only a small part of the horticultural novelties are real mutations, although they do occur from time to time. If useful, they are as a rule isolated and multiplied, and if necessary, improved by selection. They are in many instances, as constant from seed as the unavoidable influence of vicinism allows them to be. Exact observations on the origin, or on the degree of constancy, are usually lacking, [609] the notes being ordinarily made for commercial purposes, and often only at the date of introduction into trade, when the preceding stages of the novelty may have been partly forgotten.With this necessary prelude I will now give a condensed survey of the historical facts relating to the origin of new horticultural varieties. An ample description has been given recently by Korshinsky, a Russian writer, who has brought together considerable historical material as evidence of the sudden appearance of novelties throughout the whole realm of garden plants.The oldest known, and at the same time one of the most accurately described mutations is the origin of the cut-leaved variety of the greater celandine orChelidonium majus. This variety has been described either as such, or as a distinct species, calledChelidonium laciniatumMiller.It is distinguished from the ordinary species, by the leaves being cut into narrow lobes, with almost linear tips, a character which is, as we have seen on a previous occasion, repeated in the petals. It is at present nearly as commonly cultivated in botanical gardens as theC. majus, and has escaped in many localities and is observed to thrive as readily as the native wild [610] plants. It was not known until a few years before the close of the 16th century. Its history has been described by the French botanist, Rose. It was seen for the first time in the garden of Sprenger, an apothecary of Heidelberg, where theC. majushad been cultivated for many years. Sprenger discovered it in the year 1590, and was struck by its peculiar and sharply deviating characters. He was anxious to know whether it was a new plant and sent specimens to Clusius and to Plater, the last of whom transmitted them to Caspar Bauhin. These botanists recognized the type as quite new and Bauhin described it some years afterwards in his Phytopinax under the name ofChelidonium majus foliis quernis, or oak-leaved celandine. The new variety soon provoked general interest and was introduced into most of the botanical gardens of Europe. It was recognized as quite new, and repeated search has been made for it in a wild state, but in vain. No other origin has been discovered than that of Sprenger's garden. Afterwards it became naturalized in England and elsewhere, but there is not the least doubt as to its derivation in all the observed cases.Hence its origin at Heidelberg is to be considered as historically proven, and it is of course only legitimate to assume that it originated in [611] the year 1590 from the seeds of theC. majus. Nevertheless, this was not ascertained by Sprenger, and some doubt as to a possible introduction from elsewhere might arise. If not, then the mutation must have been sudden, occurring without visible preparation and without the appearance of intermediates.From the very first, the cut-leaved celandine has been constant from seed. Or at least it has been propagated by seed largely and without difficulty. Nothing, however, is known about it in the first few years of its existence. Later careful tests were made by Miller, Rose and others and later by myself, which have shown its stability to be absolute and without reversion, and it has probably been so from the beginning. The fact of its constancy has led to its specific distinction by Miller, as varieties were in his time universally, and up to the present time not rarely, though erroneously, believed to be less stable than true species.Before leaving the laciniate celandine it is to be noted that in crosses withC. majusit follows the law of Mendel, and for this reason should be considered as a retrograde variety, the more so, as it is also treated as such from a morphological point of view by Stahl and others.We now come to an enumeration of those cases in which the date of the first appearance [612] of a new horticultural variety has been recorded, and I must apologize for the necessity of again quoting many variations, which have previously been dealt with from another point of view. In such cases I shall limit myself as closely as possible to historical facts. They have been recorded chiefly by Verlot and Carriere, who wrote in Paris shortly after the middle of the past century, and afterwards by Darwin, Korshinsky, and others. It is from their writings and from horticultural literature at large that the following evidence is brought together.A very well-known instance is that of the dwarf variety ofTagetes signata, which arose in the nursery of Vilmorin in the year 1860. It was observed for the first time in a single individual among a lot of the ordinaryTagetes signata. It was found impossible to isolate it, but the seeds were saved separately. The majority of the offspring returned to the parental type, but two plants were true dwarfs. From these the requisite degree of purity for commercial purposes was reached, the vicinists not being more numerous than 10% of the entire number. The same mutation had been observed a year earlier in the same nursery in a lot ofSaponaria calabrica. The seeds of this dwarf repeated the variety in the next generation, but in the third none were observed. Then the variety was [613] thought to be lost, and the culture was given up, as the Mendelian law of the splitting of varietal hybrids was not known. According to our present knowledge we might expect the atavistic descendants of the first dwarf to be hybrids, and to be liable to split in their progeny into one-fourth dwarfs and three-fourths normal specimens. From this it is obvious that the dwarfs would have appeared a second time if the strain had been continued by means of the seeds of the vicinistic progeny.In order to avoid a return to this phase of the question, another use of the vicinists should at once be pointed out. It is the possibility of increasing the yield of the new variety. If space admits of sowing the seeds of the vicinists, a quarter of the progeny may be expected to come true to the new type, and if they were partly pollinated by the dwarfs, even a larger number would do so. Hence it should be made a rule to sow these seeds also, at least when those of the true representatives of the novelty do not give seed enough for a rapid multiplication.Other dwarfs are recorded to have sprung from species in the same sudden and unexpected manner, as for instanceAgeratum coeruleumof the same nursery, furtherClematis Viticella nanaandAcer campestre nanum.Prunus Mahaleb nanawas discovered in 1828 in one [614] specimen near Orleans by Mme. LeBrun in a large culture of Mahaleb.Lonicera tatarica nanaappeared in 1825 at Fontenay-aux Roses. A tall variety of the strawberry is called "Giant of Zuidwijk" and originated at Boskoop in Holland in the nursery of Mr. van de Water, in a lot of seedlings of the ordinary strawberry. It was very large, but produced few runners, and was propagated with much difficulty, for after six years only 15 plants were available. It proved to be a late variety with abundant large fruit, and was sold at a high price. For a long time it was prominent in cultures in Holland only.Varieties without prickles are known to have originated all of a sudden in sundry cases.Gleditschia sinensis, introduced in 1774 from China, gave two seedlings without spines in the year 1823, in the nursery of Caumzet. It is curious in being one of the rare instances where a simultaneous mutation in two specimens is acknowledged, because as a rule, such records comply with the prevailing, though inexact, belief that horticultural mutations always appear in single individuals.From Korshinsky's survey of varieties with cut leaves or laciniate forms the following cases may be quoted. In the year 1830 a nurseryman named Jacques had sown a large lot of elms, [615]Ulmus pedunculata. One of the seedlings had cut leaves. He multiplied it by grafting and gave it to the trade under the name ofU. pedunculata urticaefolia. It has since been lost.Laciniate alders seem to have been produced by mutation at sundry times. Mirbel says that theAlnus glutinosa laciniatais found wild in Normandy and in the forests of Montmorency near Paris. A similar variety has been met with in a nursery near Orleans in the year 1855. In connection with this discovery some discussion has arisen concerning the question whether it was probable that the Orleans strain was a new mutation, or derived in some way from the trees cited by Mirbel. Of course, as always in such cases, any doubt, once pronounced, affects the importance of the observation for all time, since it is impossible to gather sufficient historical evidence to fully decide the point. The same variety had appeared under similar circumstances in a nursery at Lyons previously (1812).Laciniated maples are said to be of relatively frequent occurrence in nurseries, among seedlings of the typical species. Loudon says that once 100 laciniated seedlings were seen to originate from seed of some normal trees. But in this case it is rather probable that the presumed [616] normal parents were in reality hybrids between the type and the laciniated form, and simply split according to Mendel's law. This hypothesis is partly founded on general considerations and partly on experiments made by myself with the cut-leaved celandine, previously alluded to, which I crossed with the type. The hybrids repeated the features of the species and showed no signs of their internal hybrid constitution. But the following year one-fourth of their progeny returned to the cut-leaved form. If the same thing has taken place in the case of Loudon's maples, but without their hybrid origin being known, the result would have been precisely what he observed.Broussonetia papyriffera dissectaoriginated about 1830 at Lyons, and a second time in 1866 at Fontenay-aux-Roses. The cut-leaved hazelnuts, birches, beeches and others have mostly been found in the wild state, as I have already pointed out in a previous lecture. A similar variety of the elder,Sambucus nigra laciniata, and its near ally,Sambucus racemosa laciniata, are often to be seen in our gardens. They have been on record since 1886 and come true from seed, but their exact origin seems to have been forgotten. Cut-leaved walnuts have been known since 1812; they come true from seed, but are extremely liable to vicinism, a nuisance which is [617] ascribed by some authors to the fact that often on the same tree the male catkins flower and fall off several weeks before the ripening of the pistils of the other form of flowers.Weeping varieties afford similar instances.Sophora japonica pendulaoriginated about 1850, andGleditschia triacanthos pendulasome time later in a nursery at Chateau-Thierry (Aisne, France). In the year 1821 the bird's cherry, orPrunus Padus, produced a weeping variety, and in 1847 the same mutation was observed for the alliedPrunus Mahaleb. Numerous other instances of the sudden origin of weeping trees, both of conifers and of others, have been brought together in Korshinsky's paper. This striking type of variation includes perhaps the best examples of the whole historical evidence. As a rule they appear in large sowings, only one, or only a few at a time. Many of them have not been observed during their youth, but only after having been planted out in parks and forests, since the weeping characters show only after several years.The monophyllous bastard-acacia originated in the same way. Its peculiarities will be dealt with on another occasion, but the circumstances of its birth may as well be given here. In 1855 in the nursery of Deniau, at Brain-sur-l'Authion (Maine et Loire), it appeared in a lot of [618] seedlings of the typical species in a single individual. This was transplanted into the Jardin des Plantes at Paris, where it flowered and bore seeds in 1865. It must have been partly pollinated by the surrounding normal representatives of the species, since the seeds yielded only one-fourth of true offspring. This proportion, however, has varied in succeeding years. Briot remarks that the monophyllous bastard acacia is liable to petaloid alterations of its stamens, which deficiency may encroach upon its fertility and accordingly upon the purity of its offspring.Broom-like varieties often occur among trees, and some are known for their very striking reversions by buds, as we have seen on a previous occasion. They are ordinarily called pyramidal or fastigiate forms, and as far as their history goes, they arise suddenly in large sowings of the normal species. The fastigiate birch was produced in this way by Baumann, theAbies concolor fastigiataby Thibault and Keteleer at Paris, the pyramidal cedar by Paillat, the analogous form ofWellingtoniaby Otin. Other instances could easily be added, though of course some of the most highly prized broom-like trees are so old that nothing is known about their origin. This, for instance, is the case with the pyramidal yew-tree,Taxus baccata fastigiata. [619] Others have been found wild, as already mentioned in a former lecture.An analogous case is afforded by the purpleleaved plums, of which the most known form is Prunus Pissardi. It is said to be a purple variety ofPrunus cerasifera, and was introduced at the close of the seventies from Persia, where it is said to have been found in Tabris. A similar variety arose independently and unexpectedly in the nursery of Spath, near Berlin, about 1880, but it seems to differ in some minor points from the Persian prototype.A white variety ofCyclamen vernummade its appearance in the year 1836 in Holland. A single individual was observed for the first time among a large lot of seedlings, in a nursery near Haarlem. It yielded a satisfactory amount of seed, and the progeny was true to the new type. Such plants propagate slowly, and it was only twenty-seven years later (1863) that the bulbs were offered for sale by the Haarlem firm of Krelage & Son. The price of each bulb was $5.00 in that year, but soon afterwards was reduced to $1.00 each, which was about thrice the ordinary price of the red variety.The firm of Messrs. Krelage & Son has brought into commerce a wide range of new bulb-varieties, all due to occasional mutations, some by seed and others by buds, or to the accidental [620] transference of new qualities into the already existing varieties by cross-pollination through the agency of insects. Instead of giving long lists of these novelties, I may cite the black tulips, which cost during the first few years of their introduction about $25.00 apiece.Horticultural mutations are as a rule very rare, especially in genera or species which have not yet been brought to a high degree of variability. In these the wide range of varieties and the large scale in which they are multiplied of course give a greater chance for new varieties. But then the possibilities of crossing are likewise much larger, and apparent changes due to this cause may easily be taken for original mutations.The rarity of the mutations is often proved by the lapse of time between the introduction of a species and its first sport. Some instances may be given. They afford a proof of the length of the period during which the species remained unaltered, although some of these alterations may be due to a cross with an allied form.Erythrina Crista-galliwas introduced about 1770, and produced its first sport in 1884, after more than a century of cultivation.Begonia semperflorenshas been cultivated since 1829, and for half a century before it commenced sporting. The same length of time has elapsed [621] between the first culture and the first variation ofCrambe maritima. Other cases are on record in which the variability exhibited itself much sooner, perhaps within a few years after the original discovery of the species. But such instances seem, as a rule, to be subject to doubt as to the concurrence of hybridization. So for instance theIris lortetii, introduced in the year 1895 from the Lebanon, which produced a white variety from its very first seeds. If by chance the introduced plants were natural hybrids between the species and the white variety, this apparent and rather improbable mutation would find a very simple explanation. The length of the period preceding the first signs of variability is largely, of course, due to divergent methods of culture. Such species asErythrina, which are perennial and only sown on a small scale, should not be expected to show varieties very soon. Annual species, which are cultivated yearly in thousands or even hundreds of thousands of individuals, have a much better chance. Perhaps the observed differences are largely due to this cause.Monstrosities have, from time to time, given rise to cultivated races. The cockscomb orCelosiais one of the most notorious instances. Cauliflowers, turnips and varieties of cabbages are recorded by De Candolle to have arisen in [622] culture, more than a century ago, as isolated monstrous individuals. They come true from seed, but show deviations from time to time which seem to be intimately linked with their abnormal characters. Apetalous flowers may be considered as another form of monstrosity, and inSalpiglossis sinuatasuch a variety without a corolla made its appearance in the year 1892 in the nursery of Vilmorin. It appeared suddenly, yielded a good crop of seed and was constant from the outset, without any sign of vicinism or impurity.In several cases the origin of a variety is obscure, while the subsequent historical evidence is such as to make an original sudden appearance quite probable. Although these instances offer but indirect evidence, and will sooner or later lose their importance, it seems desirable to lay some stress on them here, because most of these cases are very obvious and more striking than purely historical facts. Sterile varieties belong to this heading. Sometimes they bear fruit without kernels, sometimes flowers without sexual organs, or even no flowers at all. Instances have been given in the lecture on retrograde varieties; they are ordinarily assumed to have originated by a leap, because it is not quite clear how a loss of the capacity for the formation of seeds could have been slowly accumulated [623] in preceding generations. An interesting case is afforded by a sterile variety of corn, which originated some time ago in my own pedigree-cultures made for another purpose, and which had begun with an ear of 1886. The first generation from the original seeds showed nothing particular, but the second at once produced quite a number of sterile plants. The sterility was caused by the total lack of branches, including those bearing the pistillate flowers. The terminal spikes themselves were reduced to naked spindles, without branches, without flowers and even almost without bracts.In some individuals, however, this negative character was seen to give way at the tip, showing a few small naked branches. Of course it was impossible to propagate this curious form, but my observations showed that it sprang into existence from known ancestors by a single step or sudden leap. This leap, however, was not confined to a single specimen; on the contrary it affected 40 plants out of a culture of 340 individuals. The same phenomenon was repeated from the seeds of the normal plants in the following year, but afterwards the monstrosity disappeared.The Italian poplar affords another instance. It is considered by some authors as a distinct species,Populus italica, and by others as a [624] broom-like variety of thePopulus nigra, from which it is distinguished by its erect branches and other characters of minor importance. It is often called the pyramidal or fastigiate poplar. Its origin is absolutely unknown and it occurs only in the cultivated state. In Italy it seems to have been cultivated from the earliest historical times, but it was not introduced into other countries till the eighteenth century. In 1749 it was brought into France, and in 1758 into England, and to day it may be seen along roads throughout central Europe and in a large part of Asia. But the most curious fact is that it is only observed in staminate specimens; pistillate trees have not been found, although often sought for. This circumstance makes it very probable that the origin of the broom-like poplar was a sudden mutation, producing only one individual. This being staminate, it has been propagated exclusively by cuttings. It is to be admitted, however, that no material evidence is at hand to prove that it is not an original wild species, the pistillate form of which has been lost by vegetative multiplication. One form only of many dioecious plants is to be found in cultivation, as, for instance some South American species ofRibes.Total lack of historical evidence concerning [625] the origin of a variety has sometimes been considered as sufficient proof of a sudden origin. The best known instance is that of the renowned cactus-dahlia with its recurved instead of incurved ray-florets. It was introduced from Mexico into the Netherlands by Van den Berg of Jutphaas, under the following remarkable circumstances. In the autumn of 1872 one of his friends had sent him a small case, containing seeds, bulbs and roots from Mexico. From one of these roots aDahliashoot developed. It was cultivated with great care and bloomed next year. It surprised all who saw it by the unexpected peculiarity of its large rich crimson flowers, the rays of which were reversed tubular. The margins of the narrow rays were curved backwards, showing the bright color of the upper surface. It was a very showy novelty, rapidly multiplied by cuttings, and was soon introduced into commerce. It has since been crossed with nearly all other available varieties of theDahlia, giving a large and rich group of forms, bound together by the curious curling of the petals. It has never been observed to grow in Mexico, either wild or in gardens, and thus the introduced individual has come to be considered as the first of its race.I have already mentioned that the rapid production of large numbers of new varieties, by [626] means of the crossing of the offspring of a single mutant with previously existing sorts, is a very common feature in horticultural practice. It warns us that only a small part of the novelties introduced yearly are due to real mutations. Further instances of novelties with such a common origin are the purple-leaved dahlias, the gooseberries without prickles, the double petunias, erect gloxinias and many others. Accumulation of characters, acquired in different races of a species, may easily be effected in this way; in fact it is one of the important factors in the breeding of horticultural novelties.I have alluded more than once in this lecture to the question, whether it is probable that mutations occur in one individual or in more. The common belief among horticulturists is that, as a rule, they appear in a single plant. This belief is so widespread that whenever a novelty is seen for the first time in two or more specimens it is at once suggested that it might have originated and been overlooked in a previous generation. Not caring to confess a lack of close observation, the number of mutants in such cases is usually kept secret. At least this statement has been made to me by some of the horticulturists at Erfurt, whom I visited some years ago in order to learn as much as [627] possible about the methods of production of their novelties. Hence it is simply impossible to decide the question on the basis of the experience of the breeders. Even in the case of the same novelty arising in sundry varieties of the same species, the question as to common origin, by means of crossing, is often hard to decide, as for instance in moss-roses and nectarines. On the other hand, instances are on record where the same novelty has appeared at different times, often at long intervals. Such is the case with the butterfly-cyclamen, a form with wide-spreading petals which originated in Martin's nursery in England. The first time it was seen it was thought to be of no value, and was thrown away, but when appearing for a second time it was multiplied and eventually placed on the market. Other varieties ofCyclamen, as for instance the crested forms, are also known to have originated repeatedly.In concluding this series of examples of horticultural mutations, I might mention two cases, which have occurred in my own experimental garden. The first refers to a tubularDahlia. It has ray-florets, the ligules of which have their margins grown together so as to form tubes, with the outer surface corresponding to the pale under-surface of the corolla.This novelty originated in a single plant in a [628] culture from the seed of the dwarf variety "Jules Chretien." The seeds were taken from introduced plants in my garden, and as the sport has no ornamental value it is uncertain whether this was the first instance or whether it had previously occurred in the nursery at Lyons, from whence the bulbs were secured. Afterwards it proved true from seed, but was very variable, exhibiting rather the features of an ever-sporting variety.Another novelty was seen the first time in several individuals. It was a pink sport of the European cranesbill,Geranium pratense. It arose quite unexpectedly in the summer of 1902 from a striped variety of the blue species. It was seen in seven specimens out of a lot of about a hundred plants. This strain was introduced into my garden in 1897, when I bought two plants under the name ofGeranium pratense album, which however proved to belong to the striped variety. From their seeds I sowed in 1898 a first generation, of which a hundred plants flowered the next year, and from their seeds I sowed in 1900 the lot which produced the sport. Neither the introduced plants nor their offspring had exhibited the least sign of a color-variation, besides the blue and white stripes. Hence it is very probable that my novelty was a true first mutation, the more probably [629] so since a pink variety would without doubt have a certain horticultural value and would have been preserved if it had occurred. But as far as I have been able to ascertain, it is as yet unknown, nor has it been described until today.Summing up the results of this long, though very incomplete, list of horticultural novelties with a more or less well-known origin, we see that sudden appearances are the rule. Having once sprung into existence the new varieties are ordinarily constant, except as affected by vicinism. Details concerning the process are mostly unavailable or at least are of very doubtful value. And to this it should be added that really progressive mutations have hardly been observed in horticulture. Hence the theoretical value of the facts is far less than might have been expected.
[630]
LECTURE XXIISYSTEMATIC ATAVISM
LECTURE XXIISYSTEMATIC ATAVISM
SYSTEMATIC ATAVISM
The steady cooperation of progression and retrogression is one of the important principles of organic evolution. I have dwelt upon this point more than once in previous lectures. I have tried to show that both in the more important lines of the general pedigree of the vegetable kingdom, and in the numerous lateral branches ending in the genera and species within the families, progression and retrogression are nearly always at work together. Your attention has been directed to the monocotyledons as an example, where retrogression is everywhere so active that it can almost be said to be the prevailing movement. Reduction in the vegetative and generative organs, in the anatomical structure and growth of the stems, and in sundry other ways is the method by which the monocotyledons have originated as a group from their supposed ancestors among the lower dicotyledonous families. Retrogression is the leading idea in the larger families of the group, [631] as for instance in the aroids and the grasses. Retrograde evolution is also typical in the highest and most highly differentiated family of the monocotyledons, the orchids, which have but one or two stamens. In the second place I have had occasion more than once to assert that retrogression, though seemingly consisting in the disappearance of some quality, need not, as a rule, be considered as a complete loss. Quite on the contrary, it is very probable that real losses are extremely rare, if not wholly lacking. Ordinarily the loss is only apparent, the capacity becomes inactive only, but is not destroyed. The character has become latent, as it is commonly stated, and therefore may return to activity and to the full display of its peculiarity, whenever occasion offers.Such a return to activity was formerly called atavism. But as we have seen, when dealing with the phenomena of latency at large, sundry cases of latency are to be distinguished, in order to get a clear insight into these difficult processes.So it is with atavism, too. If any plant reverts to a known ancestor, we have a positive and simple case. But ancestors with alternate specific marks are as a rule neither historically nor experimentally manifest. They are only reputed to be such, and the presumption rests [632] upon the systematic affinity between the derivative species and its nearest probable allies. Such reversions are now to be examined at some length and may be adequately treated under the head of systematic atavism. To this form of atavism pertain, on the basis of our definition, those phenomena by which species assume one or more characters of allies, from which they are understood to have descended by the loss of the character under discussion. The phenomena themselves consist in the production of anomalies and varieties, and as the genetic relation of the latter is often hardly beyond doubt, the anomalies seem to afford the best instances for the study of systematic atavism. This study has for its chief aim the demonstration of the presence of the latent characters, and to show that they return to activity suddenly and not by a slow and gradual recovery of the former features. It supports the assertion that the visible elementary characters are essentially an external display of qualities carried by the bearers of heredity, and that these bearers are separate entities, which may be mingled together, but are not fused into a chaotic primitive life-substance. Systematic atavism by this means leads us to a closer examination of the internal and concealed causes, which rule the affinities and divergencies of [633] allied species. It brings before us and emphasizes the importance of the conception of the so-called unit-characters.The primrose will serve as an example. In the second lecture we have seen that the old species of Linnaeus, thePrimula veris, was split up by Jacquin into three smaller ones, which are calledP. officinalis,P. elatiorandP. acaulis. From this systematic treatment we can infer that these three forms are assumed to be derived from a common ancestor. Now two of them bear their flowers in bracted whorls, condensed into umbels at the summits of a scape. The scapes themselves are inserted in the axils of the basal leaves, and produce the flowers above them. In the third species,Primula acaulis, this scape is lacking and the flowers are inserted singly in the axils on long slender stalks. For this reason the species is called acaulescent, indicating that it has no other stem than the subterranean rootstock. But on closer inspection we observe that the flower stalks are combined into little groups, each group occupying the aril of one of the basal leaves. This fact at once points to an analogy with the umbellate allies, and induces us to examine the insertion of the flowers more critically. In doing so we find that they are united at their base so as to constitute a sessile umbel. [634] The scapes are not absolutely lacking, but only reduced to almost invisible rudiments.Relying upon this conclusion we infer that all of the three elementary species have umbels, some pedunculate and the others not. On this point they agree with the majority of the allied species in the genus and in other genera, as for instance inAndrosace. Hence the conclusion that the common ancestors were perennial plants with a rootstock bearing their flowers in umbels or whorls on scapes. Lacking in thePrimula veris, these scapes must obviously have been lost at the time of the evolution of this form.Proceeding on this line of speculation we at once see that a very adequate opportunity for systematic atavism is offered here. According to our general conception the apparent loss of a scape is no proof of a corresponding internal loss, but might as well be caused simply by the reduction of the scape-growing capacity to a latent or inactive state. It might be awakened afterwards by some unknown agency, and return to activity.Now this is exactly what happens from time to time. In Holland the acaulescent primrose is quite a common plant, filling the woods in the spring with thousands of clusters of bright yellow flowers. It is a very uniform type, but in [635] some years it is seen to return to atavistic conditions in some rare individuals. More than once I have observed such cases myself, and found that the variation is only a partial one, producing one or rarely two umbels on the same plant, and liable to fail of repetition when the varying specimens are transplanted into the garden for further observation. But the fact remains that scapes occur. The scapes themselves are of varying length, often very short, and seldom long, and their umbels display the involucre of bracts in a manner quite analogous to that of thePrimula officinalisandP. elatior. To my mind this curious anomaly strongly supports the view of the latent condition of the scape in the acaulescent species, and that such a dormant character must be due to a descent from ancestors with active scapes, seems to be in no need of further reiteration. Returning to activity the scapes at once show a full development, in no way inferior to that of the allied forms, and only unstable in respect to their length.A second example is afforded by the bracts of the crucifers. This group is easily distinguished by its cruciform petals and the grouping of the flowers into long racemes. In other families each flower of such an inflorescence would be subtended by a bract, according to the [636] general rule that in the higher plants side branches are situated in the arils of leaves. Bracts are reduced leaves, but the spikes of the cruciferous plants are generally devoid of them. The flower-stalks, with naked bases, seem to arise from the common axis at indefinite points.Hence the inference that crucifers are an exception to a general rule, and that they must have originated from other types which did comply with this rule, and accordingly were in the possession of floral bracts. Or, in other words, that the bracts must have been lost during the original evolution of the whole family. This conclusion being accepted, the accidental re-apparition of bracts within the family must be considered as a case of systematic atavism, quite analogous to the re-appearance of the scapes in the acaulescent primrose. The systematic importance of this phenomenon, however, is far greater than in the first case, in which we had only to deal with a specific character, while the abolition of the bracts has become a feature of a whole family.This reversion is observed to take place according to two widely different principles. On one hand, bracts may be met with in a few stray species, assuming the rank of a specific character. On the other hand they may be seen [637] to occur as an anomaly, incompletely developed, often very rare and with all the appearance of an accidental variation, but sometimes so common as to seem nearly normal.Coming now to particular instances, we may turn our attention in the first place to the genusSisymbrium. This is a group of about 50 species, of wide geographic distribution, among which the hedge mustard (S. officinalis) is perhaps the most common of weeds. Two species are reputed to have bracts,Sisymbrium hirsutumandS. supinum. Each flower-stalk of their long racemes is situated in the aril of such a bract, and the peculiarity is quite a natural one, corresponding exactly to what is seen in the inflorescence of other families. Besides theSisymbriumsome six other genera afford similar structures.Erucastrum pollichiihas been already alluded to in a former lecture when dealing with the same problem from another point of view. As previously stated, it is one of the most manifest and most easily accessible examples of a latent character becoming active through systematic atavism. In fact, its bracts are found so often as to be considered by some authors as of quite normal occurrence. Contrasted with those of the above mentioned species ofSisymbrium, they are not seen at the base of all the flower [638] stalks, but are limited to the lowermost part of the raceme, adorning a few, often ten or twelve, and rarely more flower-stalks. Moreover they exhibit a feature which is indicative of the presence of an abnormality. They are not all of the same size, but decrease in length from the base of the raceme upward, and finally slowly disappear.Besides these rare cases there are quite a number of cruciferous species on record, which have been observed to bear bracts. Penzig in his valuable work on teratology gives a list of 33 such genera, many of them repeating the anomaly in more than one species. Ordinary cabbages are perhaps the best known instance, and any unusual abundance of nourishment, or anomalous cause of growth seems to be liable to incite the development of bracts. The hedge garlic or garlic mustard (Alliaria), the shepherd's purse, the wormseed orErysimum cheiranthoidesand many others afford instances. In my cultures of Heeger's shepherd's purse, the new species derived at Landau in Germany from the common shepherd's purse, the anomaly was observed to occur more than once, showing that the mutation, which changed the fruits, had not in the least affected this subordinate anomalous peculiarity. In all these cases the bracts behave as with the Erucastrum, [639] being limited to the base of the spike, and decreasing in size from the lower flowers upward. Connected with these atavistic bracts is a feature of minor importance, which however, by its almost universal accompaniment of the bracts, deserves our attention, as it is indicative of another latent character. As a rule, the bracts are grown together with their axillary flower-stalk. This cohesion is not complete, nor is it always developed in the same degree. Sometimes it extends over a large part of the two organs, leaving only their tips free, but on other occasions it is limited to a small part of the base. But it is very interesting that this same cohesion is to be seen in the shepherd's purse, in the wormseed and in the cabbage, as well as in the case of theErucastrumand most of the other observed cases of atavistic bracts. This fact suggests the idea of a common origin for these anomalies, and would lead to the hypothesis that the original ancestors of the whole family, before losing the bracts, exhibited this peculiar mode of cohesion.Bracts and analogous organs afford similar cases of systematic atavism in quite a number of other families. Aroids sometimes produce long bracts from various places on their spadix, as may be seen in the cultivated greenhouse species,Anthurium scherzerianum. [640] Poppies have been recorded to bear bracts analogous to the little scales on the flower-stalks of the pansies, on the middle of their flower stalks. A similar case is shown by the yellow foxglove orDigitalis parviflora. The foxgloves as a rule have naked flower-stalks, without the two little opposite leafy organs seen in so many other instances. The yellow species, however, has been seen to produce such scales from time to time. The honeysuckle genus is, as a rule, devoid of the stipules at the base of the petiole, butLonicera etruscahas been observed to develop such organs, which were seen to be free in some, but in other specimens were adnate to the base of the leaf, and even connate with those of the opposite leaf.Other instances could be given proving that bracts and stipules, when systematically lacking, are liable to reappear as anomalies. In doing so, they generally assume the peculiar characters that would be expected of them by comparison with allied genera in which they are of normal occurrence. There can be no doubt that their absence is due to an apparent loss, resulting from the reduction of a formerly active quality to inactivity. Resuming this effective state, the case attains the value and significance accorded to systematic atavism.A very curious instance of reduced bracts, developing [641] to unusual size, is afforded by a variety of corn, which is calledZea Mays cryptosperma, orZea Mays tunicata. In ordinary corn the kernels are surrounded by small and thin, inconspicuous and membranaceous scales. Invisible on the integrate spikes, when ripe, they are easily detected by pulling the kernels out. Incryptospermathey are so strongly developed as to completely hide the kernels. Obviously they constitute a case of reversion to the characters of some unknown ancestor, since the corn is the only member of the grass-family with naked kernels. The var.tunicata, for this same reason, has been considered to be the original wild form, from which the other varieties of corn have originated. But as no historical evidence on this point is at hand, we must leave it as it is, notwithstanding the high degree of attractiveness attached to the suggestion.The horsetail-family may be taken as a further support of our assertion. Some species have stems of two kinds, the fertile being brownish and appearing in early spring before the green or sterile ones. In others the stems are all alike, green and crowned with a conelike spike of sporangia-bearing scales. Manifestly the dimorphous cases are to be considered as the younger ones, partly because they are obvious exceptions to the common rule, and [642] partly because the division of labor is indicative of a higher degree of evolution. But sometimes these dimorphic species are seen to revert to the primary condition, developing a fertile cone at the summit of the green summer-stem. I have had the opportunity of collecting an instance of this anomaly on the tallEquisetum telmatejain Switzerland, and other cases are on record in teratological literature. It is an obvious example of systematic atavism, occurring suddenly and with the full development of all the qualities needed for the normal production of sporangia and spores. All of these must be concealed in a latent condition within the young tissues of the green stems.More than once I have had occasion to deal with the phenomenon of torsions, as exhibited by the teasels and some other plants. This anomaly has been shown to be analogous to the cases described as double adaptations. The capacity of evolving antagonistic characters is prominent in both. The antagonists are assumed to lie quietly together while inactive. But as soon as evolution calls them into activity they become mutually exclusive, because only one of them can come to full display in the same organ. External influences decide which of the two becomes dominant and which remains dormant. This decision must take place separately [643] for each stem and each branch, but as a rule, the stronger ages are more liable to furnish anomalies than the weaker.Exactly the same thing is true of double adaptations. Every bud of the water-persicaria may develop either into an erect or into a floating stem, according as it is surrounded by water or by relatively dry soil. In other cases utility is often less manifest, but some use may either be proved, or shown to be very probable. At all events the term adaptation includes the idea of utility, and obviously useless contrivances could hardly be brought under the same head.We have also dealt with the question of heredity. It is obvious that from the flowers of the floating and erect stems of the water-persicaria seeds will result, each capable of yielding both forms. Quite the same thing was the case with the teasels. Some 40% of the progeny produce beautifully twisted stems, but whether the seed was saved from the most completely twisted specimens or from the straight plants of the race was of no importance.This phenomenon of twisting may now be considered from quite another point of view. It is a case of systematic atavism, or of the reacquirement of some ancient and long-lost quality. This quality is the alternate position of [644] the leaves, which has been replaced in the teasel family by a grouping in pairs. In order to prove the validity of this assertion, it will be necessary to discuss two points separately, viz.: relative positions of the leaves, and the manner in which the alternate position causes the stems to become twisted.Leaves are affixed to their stems and branches in various ways. Among them one is of wide occurrence throughout the whole realm of the higher plants, while all the others are more rare. Moreover these subordinate arrangements are, as a rule, confined to definite systematic groups. Such groups may be large, as for instance, the monocotyledons, that have their leaves arranged in two opposite rows in many families, or small, as genera or subdivisions of genera. Apart from these special cases the main stem and the greater part of the branches of the pedigree of the higher plants exhibit a spiral condition or a screw arrangement, all leaves being inserted at different points and on different sides of the stem. This condition is assumed to be the original one, from which the more specialized types have been derived. As is usual with characters in general, it is seen to vary around an average, the spiral becoming narrower and looser. A narrow spiral condenses the leaves, while a [645] loose one disperses them. According to such fluctuating deviations the number of leaves, inserted upon a given number of spiral circuits, is different in different species. In a vast majority of cases 13 leaves are found on 5 circuits, and as we have only to deal with this proportion in the teasels we will not consider others.In the teasels this screw-arrangement has disappeared, and has been replaced by a decussate grouping. The leaves are combined into pairs, each pair occupying the opposite sides of one node. The succeeding pairs alternate with one another, so as to place their leaves at right angles. The leaves are thus arranged on the whole stem in four equidistant rows.On the normal stem of a teasel the two members of a pair are tied to one another in a comparatively complicated way. The leaves are broadly sessile and their bases are united so as to constitute a sort of cup. The margins of these cups are bent upward, thereby enabling them to hold water, and after a rainfall they may be seen filled to the brim. It is believed that these little reservoirs are useful to the plant during the flowering period, because they keep the ants away from the honey. Considering the internal structure of the stem at the base of these cups we find that the vascular bundles of the two opposite leaves are strongly connected [646] with one another, constituting a ring which narrowly surrounds the stem, and which would impede an increase in thickness, if such were in the nature of the plant. But since the stems end their existence during the summer of their development, this structure is of no real harm.The grouping of the leaves in alternate pairs may be seen within the bud as well as on the adult stems. In order to do this, it is necessary to make transverse sections through the heart of the rosette of the leaves of the first year. If cut through the base, the pair exhibit connate wings, corresponding to the water-cups; if cut above these, the leaves seem to be free from one another.In order to compare the position of leaves of the twisted plants with this normal arrangement, the best way is to make a corresponding section through the heart of the rosette of the first year. It is not necessary to make a microscopic preparation. In the fall the changed disposition may at once be seen to affect the central leaves of the group. All the rosettes of the whole race commence with opposite leaves; those that are to produce straight stems remain in this condition, but the preparation for twisting begins at the end of the first year as shown by a special arrangement of the leaves. This [647] disposition may then be seen to extend to the very center of the rosette, by use of microscopical sections. Examining sections made in the spring, the original arrangement of the leaves of the stem is observed to continue until the beginning of the growth of the shoot. It is easy to estimate the number of leaves corresponding to a given number of spiral circuits in these sections and the proportion is found to indicate 13 leaves on 5 turns. These figures are the same as those given above for the ordinary arrangement of alternate leaves in the main lines of the pedigree of the vegetable kingdom.Leaving aside for the moment the subsequent changes of this spiral arrangement, it becomes at once clear that here we have a case of systematic atavism. The twisted teasels lose their decussation, but in doing so the leaves are not left in a disorderly dispersion, but a distinct new arrangement takes its place, which is to be assumed as the normal one for the ancestors of the teasel family. The case is to be considered as one of atavism. Obviously no other explanation is possible, than the supposition that the 5-13 spiral is still latent, though not displayed by the teasels. But in the very moment when the faculty of decussation disappears, it resumes its place, and becomes [648] as prominent as it must once have been in the ancestors, and is still in that part of their offspring, which has not become changed in this respect. Thus the proof of our assertion of systematic atavism is, in this case, not obtained by the inspection of the adult, but by the investigation of the conditions in an early stage. It remains to be explained how the twisting may finally be caused by this incipient grouping of the leaves. Before doing so, it may be as well to state that the case of the teasel is not an isolated one, and that the same conclusions are supported by the valerian, and a large number of other examples. In early spring some rosettes show a special condition of the leaves, indicating thereby at once their atavism and their tendency to become twisted as soon as they begin to expand. The Sweet William orDianthus barbatusaffords another instance; it is very interesting because a twisted race is available, which may produce thousands of instances developed in all imaginable degrees, in a single lot of plants.Viscaria oculatais another instance belonging to the same family.The bedstraw (Galium) also includes many species which from time to time produce twisted stems. I have found them myself in Holland onGalium verumandG. Aparine. Both seem [649] to be of rare occurrence, as I have not succeeded in getting any repetition by prolonged culture.Species, which generally bear their leaves in whorls, are also subjected to casual atavisms of this kind, as for instance the tall European horsetail,Equisetum Telmateja, which occasionally bears cones on its green summer stems. Its whorls are changed on the twisted parts into clearly visible spirals. The ironwood orCasuarina quadrivalvisis sometimes observed to produce the same anomaly on its smaller lateral branches.Coming now to the discussion of the way in which the twisting is the result of the spiral disposition of the leaves, we may consider this arrangement on stems in the adult state. These at once show the spiral line and it is easy to follow this line from the base up to the apex. In the most marked cases it continues without interruption, not rarely however, ending in a whorl of three leaves and a subsequent straight internode, of which there may even be two or three. The spiral exhibits the basal parts of the leaves, with the axillary lateral branches. The direction of the screw is opposed to that of the twisting, and the spiral ribs are seen to cross the line of insertion of the leaves at nearly right angles. On this line the leaves are nearer [650] to one another than would correspond to the original proportion of 5 turns for 13 leaves. In fact, 10 or even 13 leaves may not rarely be counted on a single turn. Or the twist may become so strong locally as to change the spiral into a longitudinal line. On this line all inserted leaves extend themselves in the same direction, resembling an extended flag.The spiral on the stem is simply the continuation of the spiral line from within the rosettes of the first year. Accordingly it is seen to become gradually less steep at the base. For this reason it must be one and the same with this line, and in extreme youth it must have produced its leaves at the same mutual distances as this line. Transverse sections of the growing summits of the stems support this conclusion.From these several facts we may infer that the steepness of the spiral line increases on the stem, as it is gradually changed into a screw. Originally 5 turns were needed for 13 leaves, but this number diminishes and 4 or 3 or even 2 turns may take the same number of foliar organs, until the screw itself is changed into a straight line.This change consists in an unwinding of the whole spiral, and in order to effect this the stem must become wound up in the opposite direction. The winding of the foliar screw must [651] curve the longitudinal ribs. The straighter and steeper the screw becomes, the more the ribs will become twisted. That this happens in the opposite direction is obvious, without further discussion. The twisting is the inevitable consequence of the reversal of the screw.Two points remain to be dealt with. One is the direct proof of the reversal of the screw, the other the discussion of its cause. The first may be observed by a simple experiment. Of course it proceeds only slowly, but all that is necessary is to mark the position of one of the younger leaves of a growing stem of a twisting individual and to observe the change in its position in a few hours. It will be seen to have turned some way around the stem, and finally may be seen to make a complete revolution in the direction opposite to the screw, and thereby demonstrating the fact of its uncurling.The cause of this phenomenon is to be sought in the intimate connection of the basal parts of the leaves, which we have detailed above. The fibrovascular strands constitute a strong rope, which is twisted around the stem along the line on which the leaves are inserted. The strengthening of the internodes may stretch this rope to some extent, but it is too strong to be rent asunder. Hence it opposes the normal growth, and the only manner in which the internodes [652] may adjust themselves to the forces which tend to cause their expansion is by straightening the rope. In doing so they may find the required space, by growing out in an unusual direction, bending their axes and twisting the ribs.To prove the validity of this explanation, a simple experiment may be given. If the fibrovascular rope is the mechanical impediment which hinders the normal growth, we may try the effect of cutting through this rope. By this means the hindrance may at least locally be removed. Now, of course, the operation must be made in an early stage before, or at the beginning of the period of growth, in every case before the uncurling of the rope begins. Wounds made at this time are apt to give rise to malformations, but notwithstanding this difficulty I have succeeded in giving the necessary proof. Stems operated upon become straight where the rope is cut through, though above and under the wounded part they go on twisting in the usual way.Sometimes the plants themselves succeed in tearing the rope asunder, and long straight internodes divide the twisted stems in two or more parts in a very striking manner. A line of torn leaf-bases connects the two parts of the screw and gives testimony of what has passed within [653] the tissues. At other times the straightening may have taken place directly internal to a leaf, and it is torn and may be seen to be attached to the stem by two distinct bases.Summing up this description of the hereditary qualities of our twisted teasels and of their mechanical consequences, we may say that the loss of the normal decussation is the cause of all the observed changes. This special adaptation, which places the leaves in alternating pairs, replaced and concealed the old and universal arrangement on a screw line. In disappearing, it leaves the latter free, and according to the rule of systematic atavism, this now becomes active and takes its place. If the fibrovascular connection of the leaf-bases were lost at the same time the stems would grow and become straight and tall. This change however, does not occur, and the bases of the leaves now constitute a continuous rope instead of separate rings, and thereby impede the stretching of the internodes. These in their turn avoid the difficulty by twisting themselves in a direction opposite to that of the spiral of the leaves.As a last example of systematic atavism I will refer to the reversionary changes, afforded by the tomatoes. Though the culture of this plant is a recent one, it seems to be at present in a state of mutability, producing new strains, or [654] assuming the features of their presumable ancestors. In his work "The Survival of the Unlike," Bailey has given a detailed description of these various types. Moreover, he has closely studied the causes of the changes, and shown the great tendency of the tomatoes to vicinism. By far the larger part of the observed cases of running out of varieties are caused by accidental crosses through the agency of insects. Even improvements are not rarely due to this cause. Besides these common and often unavoidable changes, others of greater importance occur from time to time. Two of them deserve to be mentioned. They are called the "Upright" and the "Mikado" types, and differ as much or even more from their parents than the latter do from any one of their wild congeners. Their characters come true from seed. The "Mikado" race or theLycopersicum grandifolium(L. latifolium) has larger and fewer leaflets than the slender and somewhat flimsy foliage of the common form. Flat or plane blades with decurrent margins constitute another character. This variety, however, does not concern our present discussion. The upright type has stiff and self-sustaining stems and branches, resembling rather a potato-plant than a tomato. Hence the nameLycopersicum solanopsisorL. validum, under which it is usually described. [655] The foliage of the plant is so distinct as to yield botanical characters of sufficient importance to justify this specific designation. The leaflets are reduced in numbers and greatly modified, and the flowers in the inflorescence are reduced to two or three. This curious race came in suddenly, without any premonition, and the locality and date of its mutation are still on record. Until some years ago it had not made its appearance for a second time. Obviously it is to be considered as a reversionary form. The limp stems of the common tomatoes are in all respects indicative of the cultivated condition. They cannot hold themselves erect, but must be tied up to supports. The color of the leaves is a paler green than should be expected from a wild plant. Considering other species of the genusSolanum, of which theLycopersicumis a subdivision, the stems are as a rule erect and self-supporting, with some few exceptions. These, however, are special adaptations as shown by the winding stems of the bitter-sweet.From this discussion we seem justified in concluding that the original appearance of the upright type was of the nature of systematic atavism. It differs however, from the already detailed cases in that it is not a monstrosity, nor an ever-sporting race, but is as constant a form [656] as the best variety or species. Even on this ground it must be considered as a representative of a separate group of instances of the universal rule of systematic reversions.Of late the same mutation has occurred in the garden of C.A. White at Washington. The parent form in this case was the "Acme," of the ordinary weak and spreading habit of growth. It is known as one of the best and most stable of the varieties and was grown by Mr. White for many years, and had not given any sign of a tendency towards change. Seeds from some of the best plants in 1899 were sown the following spring, and the young seedlings unexpectedly exhibited a marked difference from their parents. From the very outset they were more strong and erect, more compact and of a darker green than the "Acme." When they reached the fruiting stage they had developed into typical representatives of theLycopersicum solanopsisor upright division. The whole lot of plants comprised only some 30 specimens, and this number, of course, is too small to base far-reaching conclusions upon. But all of the lot showed this type, no true "Acme" being seen among them. The fruit differed in flavor, consistency and color from that of the parent, and it also ripened earlier than the latter. No seed was saved from [657] these plants, but the following year the "Acme" was sown again and found true to its type. Seeds saved from this generation in 1900 have, however, repeated the mutation, giving rise to exactly the same new upright form in 1901. This was called by its originator "The Washington." Seeds from this second mutation were kindly sent to me by Mr. White, and proved true to their type when sown in my garden.Obviously it is to be assumed in the case of the tomatoes as well as in instances from other genera cited, that characters of ancestors, which are not displayed in their progeny, have not been entirely lost, but are still present, though in a latent condition. They may resume their activity unexpectedly, and at once develop all the features which they formerly had borne.Latency, from this point of view, must be one of the most common things in nature. All organisms are to be considered as internally formed of a host of units, partly active and partly inactive. Extremely minute and almost inconceivably numerous, these units must have their material representatives within the most intimate parts of the cells.
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