CHAPTER XIIIADAPTATION AND SEED-BUOYANCY
The question of the operation of Natural Selection.—Are there two principles at work?—The presence of buoyant tissue in the seed-tests and fruit-coats of inland plants, both wild and cultivated.—Useless buoyancy.—The buoyancy of seeds and fruits is not concerned with adaptation.—Summary.
Whenwe speak of a certain structure as an adaptation to dispersal by currents through the agency of Natural Selection, it is necessary at the outset to be quite clear as to what is implied. Professor Schimper, who brought his great and varied knowledge of many other phases of plant-life to bear on this subject, is careful to clear the ground of preliminary erroneous conceptions in such a perspicuous and impartial manner that we cannot do better than follow his guidance. There are, he observes (p. 178), many mechanisms or contrivances in plants, which, though they seem to have arisen with a fixed purpose, can in no wise be regarded as having been developed for that end, since they were produced in quite a different connection and have merely acquired a new or supplementary function, of which they are the cause and not the effect.
This is very much the position that I have taken up for the whole subject of the relation between plants and their dispersing agencies, and it will be found discussed inChapter XI.It involves, as I venture to think, a dominant principle in the organic world, which it is one of the objects of this work to emphasise, namely, that Nature in dispersing plants habitually makes use of structures and capacities that were originally developed in quite another connection. Behind this change of function, this new purpose, lies the secret of the organic world. There is for me no more pregnant fact in plant-life than the thistle-seed blown before the wind, or the seed of our sea-convolvulus floating in the sea. It proves to my mind that the evolutionary power in nature is checked andhampered by conditions not of its own creation, and that two opposing forces are ever at work, the one creating and the other limiting the creative power, the actual mode of dispersal being but a blind and accidental result of the struggle.
The question of the operation of Natural Selection is not raised, as Professor Schimper indicates, until we consider whether the new function has had any bettering influence on the structure or mechanism with which it has come to be concerned. If such a modification is thus brought about it might be legitimately claimed as a result of this agency, and the term “adaptation” could be used. But if there is no evident change produced, we should be compelled to assign very subordinate limits to the capacity of Natural Selection; and in the instance of buoyant fruits and seeds it would be restricted to determining a plant’s station by the water-side and in increasing its area. It is only in the first case that we could speak of them as adaptations in the meaning attached to this term in the language of the Selection Theory. It would at first sight seem easy to ascertain whether the characters of fruits and seeds, to which the buoyancy is due, are adaptations in this sense of the word; but in reality it is far from being so. We can, however, proceed with unanimity up to a certain stage in the argument; but there agreement ends.
It has been before established that in the Pacific islands, and indeed in the tropics generally, the plants with buoyant seeds or seedvessels are mainly stationed at the coast. It has also already been shown that this littoral station is often associated with a special buoyant-tissue in the coverings of the seed or fruit; and it will now be pointed out that this tissue is, as a rule, absent or but scantily developed in the case of inland species of the same genus. Of great importance, remarks Professor Schimper (p. 179), in relation to the Selection Theory and the development of adaptations, is the comparison of the fruits and seeds of strand-plants with those of allied inland species; and he finds here evidence in support of the Darwinian view. He takes the cases of the genera Terminalia and Calophyllum, which contain both inland and littoral species; and he shows that although the same buoyant-tissue occurs in the fruit-coats of inland species, it is there much diminished, and in consequence the floating powers are considerably lessened or lost altogether (seeChapter II.). It is not pretended that this tissue has had any connection in its origin with dispersal by currents, but merely that its greater development in the shore species is an adaptation to this mode of transport.
Further testimony is adduced by this investigator (p. 182) in supporting his view in the fruits of the genera Barringtonia, Clerodendron, Cordia, and Guettarda, where the buoyant tissues extensively developed in the coast species are either non-existent or only represented by a trace in the inland species of the same genus, a difference in structure associated with the loss or great diminution of the floating capacity of the fruits concerned. I have been able to establish other examples in the cases of the genera Scævola and Tacca, which will be found referred to inChapter II.
Professor Schimper (p. 200) points to the circumstance that the “adaptations” in these fruits all belong to the diagnostic marks of the genera and the species, and contends that these plants abundantly prove the erroneous nature of the contention that Natural Selection could have played no part in the elimination of the strand-flora. My own contention is that Natural Selection has played such a part, but that in doing so it has merely availed itself of characters previously existing, without originating, modifying, or improving them in any way. The foregoing evidence might with equal fitness be employed to show, as pointed out inChapter II., that in the course of ages there has been a great sorting process by which, excluding the mangroves, plants of the xerophilous habit possessing buoyant seeds and fruits have been sorted out and placed at the coast. Direct evidence does not lead us farther than to the establishment of a littoral station for plants thus endowed. The problem whether the characters of their fruits and seeds that are concerned with buoyancy may be regarded as adaptive in the Darwinian sense lies beyond the reach of direct testimony. We can, however, approach it from the outside by several directions, and from some of these we will now proceed to deal with it.
There is first the singular circumstance that in Fiji, when the littoral plants with buoyant seeds or fruits leave the beach and extend far inland, they, as a rule, retain their floating powers and, of course, their buoyant structures. I found this to be true of Cassytha filiformis, Cerbera Odollam, Ipomœa pes capræ, Morinda citrifolia, Scævola Kœnigii, and one or two other plants mentioned inNote 44, where this subject is discussed. My experiments on these plants indicated that their fruits or seeds floated equally long, whether obtained from coast or from inland plants. This, at first sight, appears to present a serious objection to the adaptation theory; but it was not so regarded by Professor Schimper, who in a letter to me, dated March 8th, 1900, observed that he did not see “why littoral plants growing inland should losetheir adaptations to littoral life, especially if those adaptations are not conflicting with the conditions of life beyond the littoral zone, and if the competition does not require special adaptations.”
My view, however, is that any process of adaptation is unnecessary. All these plants, it is contended, were originally inland plants that acquired the buoyant qualities of their seeds and fruits in the inland stations, and ultimately found a station at the coast through the sorting process above referred to. In the case of plants like Ipomœa pes capræ and Cassytha filiformis this would be conceded, since they belong to the acknowledged non-adaptive groups discussed in the preceding chapter. It is only to some of these plants, such as Scævola Kœnigii and Cerbera Odollam, that the adaptation view of Professor Schimper is applied; and the question arises whether we are justified in making such a distinction, or, in other words, whether it is antecedently probable that two independent principles have been at work in determining the fitness of seeds and fruits for dispersal by the currents.
The plants for which the influence of adaptation through Natural Selection is claimed belong, as stated inChapter XII., almost entirely to the third group. It is admitted that with the other two groups the utmost that any sorting or selecting process would effect would be to determine a station at the coast and to extend the area of distribution. The numerical aspect of the question therefore acquires some importance; and the reader’s attention is accordingly directed to the results tabulated inNote 45, where it is shown (assuming for the time that there is no difference of opinion about the adaptive significance of the seeds and fruits concerned) that the plants of the third or adaptive group make up only about half the total. It would therefore appear that if the agencies of Natural Selection have been at work here either in bettering or in developing buoyant structures, half of the shore-plants with buoyant seeds or fruits have not come within their influence.
But the subject takes another aspect when we reflect that in some buoyant fruits, as with Ximenia americana and Calophyllum inophyllum, the two principles would seem to have been at work. Whilst from this standpoint Natural Selection is regarded as having either developed or increased in amount the layer of buoyant tissue in the fruit-coats, the buoyant kernels are not viewed as adaptive in their origin. In the case of Ximenia americana the dispersing agency of frugivorous birds adds another factor, since, as before stated, its drupes are known to be dispersed by fruit-pigeons. In the cases of Scævola Kœnigii and of Vitex trifolia,two plants belonging to the adaptive group, Professor Schimper (pp. 156, 188) admits also the dispersing agency of frugivorous birds, and he claims it for Morinda citrifolia, in the pyrenes of which he also detects a special adaptation to dispersal by currents. It may be added that, as he also points out, fruits of the non-adaptive group of littoral plants, such as Premna integrifolia (P. taitensis) and Cassytha filiformis, would sometimes also attract birds. In fact, those of the last-named have been found in the crops of pigeons (Introd. Chall. Bot., p. 46).
Looking at all these littoral plants with fruits that are equally fitted for dispersal by birds and by currents, we may now ask, Where does the general principle of adaptation to dispersal lie? Whatever view we adopt, we must apply the same view to all, whether it be a question of dispersal by birds or by currents. We cannot choose between two sets of principles determining the buoyancy of seeds and fruits any more than we can regard a fleshy drupe and a buoyant seed as illustrating different principles regulating the dispersal of plants. Nature works with uniformity in these matters, and if the Natural Selection theory is held to explain one case it ought to account for all. Yet nobody would go so far as this; and this view of dispersal is on many grounds antecedently improbable. These difficulties disappear if we assume that in all cases the dispersing agencies have without modification made use of characters and capacities that were developed, as we now see them, in quite other connections and under quite other conditions.
It will now be necessary to look a little closer into the subject of the buoyant tissue, to the existence of which in their coats about half of the littoral plants concerned owe the floating power of their fruits or seeds. In the first place, it is to be remarked that in the case of some of the seeds of the plants of the non-adaptive groups it is also represented to a small degree in the seed-coats, although, as with Strongylodon lucidum and Mucuna urens, it is not present in sufficient amount to float the seed. In the next place, it should be noted that with some genera possessing, like Terminalia, both inland and coast species it is to be found alike in the fruit-coverings of inland and of littoral plants, though in a less degree in the case of the fruits of inland species, the floating power of which is proportionately diminished. There are, however, a few cases where this buoyant tissue is developed in inland species which belong to genera or subgenera that have no littoral species. This is what we would expect, if Natural Selection has merelyconcerned itself with placing plants of xerophilous habit possessing buoyant seeds or fruits at the coast. Under such conditions we would now and then expect to find an inland plant possessing buoyant fruits or seeds of this description that has never been able to establish itself at the coast.
A good instance is afforded by Pritchardia Gaudichaudii, a fan palm peculiar to Hawaii, the drupes of which float for several weeks and have a covering of spongy buoyant tissue (seeChapter XXV.). The seeds of Hibiscus Abelmoschus, a species distinguished subgenerically from the littoral Hibiscus tiliaceus, offer another example. They float for months, and owe their buoyancy to a layer of air-bearing tissue between the kernel and the test, in this respect differing from the seeds of the littoral species, where the floating power is due to unoccupied space in the seed-cavity. The buoyancy of the seeds of Hibiscus Abelmoschus thus offers another example of ineffectual floating power, since it is not a littoral plant, is often cultivated, and has accompanied aboriginal man over much of the tropical zone.
A singular instance of the dispersal by currents of an inland plant that occurs both wild and cultivated in tropical America, the West Indies, and on the West Coast of Africa, is afforded by Spondias lutea, Linn., which is referred to at the end ofChapter XXXII.Its “stones,” which are provided with a cork-like covering much as we find with those of Cordia subcordata and Guettarda speciosa, possess great buoyancy, and are found in the river and beach drift of those regions with the seeds in a sound condition.
A very remarkable case of ineffectual buoyancy is presented by the seedvessels of Brackenridgea, which have been found floating in the drift off the coast of New Guinea. They owe their floating power to closed cavities which would seem to arise from the failure of one of the seeds or from the abortion of an ovule. But, according to Beccari, their fleshy coverings would aid their dispersal by frugivorous birds; and since the species are all much localised and are rarely littoral in their habit, it is very probable that birds have mainly effected the dispersal of the genus (seeNote 46). It has, however, been shown in the previous chapter that Premna taitensis and Morinda citrifolia owe their dispersal by currents to similar cavities in the seeds or “stones.”
Amongst the inland plants possessing seeds or fruits that are dispersed by the currents without aiding the distribution of the species may be recognised types of both the adaptive and non-adaptive groups. A singular instance is afforded by the largeseeds almost an inch long of a huge pumpkin (Cucurbita) which, in sound condition, form one of the commonest constituents of the beach drift on the coast of Chile from Valparaiso northward to Iquique. The fruit is commonly eaten by the lower classes. The seeds, which are very buoyant, contain a kernel that does not float, the buoyancy being due to the water-tight coats which, as shown in the plate inChapter XII., possess well developed air-bearing tissues. It may here be observed that Martins refers to the germination of seeds of Cucurbita pepo after 45 and 93 days’ flotation in sea-water.
One sometimes finds buoyant tissue developed in the seeds of bottle-gourds, where it can serve no useful purpose of dispersal. Thus small bottle-gourds, seemingly of the genus Cucurbita rather than of Lagenaria, are to be commonly found afloat in the Guayaquil River and stranded on the Ecuador beaches. They will float for many months, and contain the seeds dried up into a small loose compacted mass in their interior. These seeds, which contain a layer of spongy air-bearing tissue in their coverings, will in several cases float for months. Some that I had been keeping two months afloat in sea-water germinated freely. It is shown inNote 47that bottle-gourds containing sound seeds are dispersed far and wide by the currents. In some species the seeds are buoyant, and in others they sink in sea-water; but the gourds themselves will float for probably a year or more, and the floating capacity of the seeds when it exists is too insignificant to affect the fruit’s buoyancy.
Other instances of the useless buoyancy of fruits of inland plants are afforded by different species of Citrus. In the floating drift of the Fijian rivers the fruits of the wild and indigenous Shaddock (C. decumana) and of an inedible Orange, also wild and indigenous (C. vulgaris?), are at certain times to be found, the latter often in numbers. The first-named floats four to five weeks in sea-water, and the last-named nearly two months, and both are to be observed floating out at sea between the islands. The fruits of the Tahitian Orange, a variety of C. aurantium, floated in sea-water between three and four weeks. The seeds of these and other species of Citrus sank in from a few hours to a day or two. The buoyancy of the fruit depends on the rind—the thicker the rind the greater the floating power. This was not only shown in the length of the period of flotation, but also in the buoyant behaviour of the fruit. With the Tahitian Orange, where the rind is relatively thin, the fruits floated heavily in sea-water and only protruded slightly above the surface. With the Shaddock andwith the other indigenous species of Citrus, the fruits floated lightly and protruded half-way out of the water.
There is nothing trivial in these examples of buoyant fruits. That they have at times aided in the dispersal of the genus, with man’s assistance in planting the seeds of the stranded fruits, I cannot doubt; but unaided by man such buoyant capacities would be useless for purposes of effective dispersal by currents. Between the two genera Terminalia and Citrus there is this great distinction, that the former is more or less halophilous, some of its species being at home on the sea-beaches, whilst the latter, as Schimper would term it, is salt-shy, and includes no halophytes or plants of the sea-shore amongst its species. The only effect of buoyancy of the fruits on the distribution of the species of Citrus would be to place them by the side of the river and the pond. This has evidently been its result in the case of the Shaddock in Fiji, where, as Seemann remarks, it often thickly lines the banks of the rivers.
As also indicating that the buoyancy of the seed or fruit would never, apart from the halophilous habit, endow an inland plant with a littoral station, the examples of the Oak (Quercus robur) and of the Hazel (Corylus avellana) may be taken. As shown inNote 48, these fruits acquire floating power by drying, on account of the space formed by the shrinking of the kernel. They occur commonly in beach drift, but rarely in a sound condition; yet experiment has proved that they will sometimes germinate after prolonged sea-water flotation. The fruits of other species of Quercus are also transported in tropical regions by the currents, but never, as far as I could learn, effectively. The Amentaceæ as an order are “salt-shy,” and with only a few exceptions shun the sea-beach.
In the great sorting-process, by which xerophytic plants with buoyant seeds or fruits have been placed at the coast, and hygrophytic plants with similar fruits or seeds have been stationed at the riverside or by ponds and lakes, one might expect to find that other influences may have at times been in conflict with the selecting operation here indicated. To this cause may probably be attributed the cases of “useless buoyancy” above referred to. Here we find in some inland plants fruits and seeds with buoyant tissues in their coverings that in the case of littoral plants would have been regarded as the result of adaptation to dispersal by currents. Such cases go to emphasize the conclusion already indicated that these tissues could not have been developed through the agency of Natural Selection. But the great objection againstthe application of the Darwinian view to the general subject of the buoyancy of the seeds and fruits of littoral plants lies in the circumstance that quite half of the plants concerned are admitted to be outside the scope of the theory, and that for these another explanation has to be found. I think we may fairly claim that in a matter which finally resolves itself into a question of buoyancy one explanation should cover all. We have thus to decide whether to regard as adaptations to dispersal by currents the structures of the buoyant seeds and fruits of littoral plants; or whether to hold the view that as far as dispersal by currents is concerned such structures are purely accidental, and that Nature has never directly concerned herself in the matter at all. The first explanation lies under the disadvantage above alluded to, and it remains to be learned whether the second view could be made to cover all cases of dispersal by currents. Further investigation on many points is yet required; but, apart from the evidence against Natural Selection as the principal agency that has been produced in this chapter, a powerful argument in favour of the view that the buoyancy of seeds and fruits is not concerned with adaptation is, that as a rule the floating capacity of the seed or fruit has no direct relation with the density of sea-water. Generally speaking, as shown inChapter X., these seeds and fruits are much more buoyant than they need to be, that is to say, if they owe their floating power to adaptation to dispersal by currents. This is quite in accordance with the argument developed inChapter XI.with regard to the general question of plant-distribution, that dispersing agencies make use of characters and capacities of seeds and fruits that were never intended for them.
(1) There are many mechanisms or contrivances in plants that now serve a purpose for which they were not originally developed.
(2) Of this nature, it is contended, is the relation between fruits and seeds and the agencies of dispersal.
(3) If, however, the structure or mechanism is made more effective by the new function, such a modification may be regarded as an “adaptation” in the language of the theory of Natural Selection.
(4) It is held by Professor Schimper that the structures connected with the buoyancy of the fruits or seeds of several tropical littoral plants are, in the above sense, adaptations; and he points to several genera where the buoyant tissues in the coverings of the fruits or seeds of the coast species are scantily represented or absent in the inland species of the same genus, a difference corresponding with the loss or diminution of the floating powers.
(5) This contrast in structure and in floating capacity between the fruits or seeds of inland and coast species of the same genus is beyond dispute, and the author adduces fresh data in support of it.
(6) But he contends that it is not proved that the relatively great development of buoyant tissues in the case of littoral plants is the effect of adaptation; and that if the selecting process had been confined to sorting out the xerophilous plants with buoyant seeds or fruits and to placing them at the coast, the same contrast would have been produced.
(7) In support of this contention he points out that when such littoral plants extend inland the floating capacity and the buoyant tissues are as a rule retained; and that in those exceptional cases where inland plants possess buoyant fruits or seeds these tissues are sometimes well developed under conditions in which they could never aid the plant’s dispersal.
(8) But the most serious objection against the adaptation view is that admittedly only about half of the shore-plants with buoyant fruits or seeds come within its scope. Therefore a second explanation has to be framed for the other plants concerned.
(9) As showing the difficulties raised by regarding some of the structures connected with buoyancy as “adaptive” and others as “accidental,” it is pointed out that some fruits possess the two kinds of structure. It is also shown that in several cases fruits endowed with buoyant tissues are just as well adapted for dispersal by frugivorous birds; and the instance of Ximenia americana is cited where a drupaceous fruit, known to be dispersed by fruit-pigeons, possesses also in its “stone” both the “adaptive” and “non-adaptive” types of “buoyant structures.”
(10) It is urged that whatever is the relation between the buoyancy of the seeds and fruits of shore-plants and dispersal by currents, there has been a uniform principle affecting all.
(11) The weight of evidence is regarded as adverse to the Natural Selection theory, an inference which is consistent with theconclusion arrived at inChapter X.that there is no direct relation between the density of sea-water and the buoyancy of seeds and fruits, the floating capacities being as a rule far greater than the adaptation view would explain. Nature, it is held, has never made any provision for dispersal by currents, the buoyancy of seeds and fruits being, as concerns the currents, a purely accidental quality.