Protective Coloration
That many species of animals are protected by their resemblance to their environment no one will probably deny. That we are ignorant in all cases as to how far this protection is necessary for the maintenance of the species must be admitted. That some of the resemblances that have been pointed out have been given fictitious value, I believe very probable.
Resemblance in color between the organism and its environment has given to the modern selectionist some of his most valuable arguments, but we should be on our guard against supposing that, because an animal may be protected by its color, the color has been acquired on this account. On the supposition that the animal has become adapted by degrees, and through selection, we meet with all the objections that have been urged, in general, against the theory of natural selection. But if we assume here also that mutations have occurred without relation to the environment, and, having once appeared, determined in some cases the distribution of the species, we have at least a simple hypothesis that appears to explain the facts. If it be claimed that the resemblance is, in some cases, too close for us to suppose that it has arisen independently of the environment, it may be pointed out that it has not been shown that such a close resemblance is at all necessary for the continued existence of the species, and hence the argument is likely to prove too much. For instance, the most remarkable case of resemblance is that of Kallima, but in the light of a recent statement by Dean it may be seriously asked whether there is absolute need of such a close resemblance to a leaf. Even if it be admitted that to a certain extent the butterfly is at times protected by its resemblance to a leaf, it is not improbable that it could exist almost equally well without such a close resemblance. If this is true, natural selection could never have brought about such a close imitation of a leaf. Cases like these of over-adaptation are not unaccountable on the theory of mutation, for on this view the adaptation may be far ahead of what the actual requirements for protection demand. We meet occasionally, I think, throughout the living world with resemblances that can have no such interpretation, and a number of the kinds of adaptations to be described in this chapter show the same relation.
Some of the cases of mimicry appear also to fall under this head; although I do not doubt that many so-called cases of mimicry are purely imaginary, in the sense that the resemblance has not been acquired on account of its relation to the animal imitated. There is no need to question that in some cases animals may be protected by their resemblance to other animals, but it does not follow, despite the vigorous assertions of some modern Darwinians, that this imitation has been the result of selection. Until it can be shown that the imitating species is dependent on its close imitation for its existence, the evidence is unconvincing; and even if, in some cases, this should prove to be the case, it does not follow that natural selection has brought about the result, or even that it is the most plausible explanation that we have to account for the results. The mutation theory gives, in such cases, an equally good explanation, and at the same time avoids some of the difficulties that appear fatal to the selection theory.
What has been said against the theory of mimicry might be repeated in much stronger terms against the hypothesis of warning colors.
It seems to me, in this connection, that the imagination of the selectionist has sometimes been allowed to “run wild”; and while it may be true that in some cases the colors may serve as a signal to the possible enemies of the animal, it seems strange that it has been thought necessary to explain the origin of such colors as the result of natural selection. Indeed, some of these warning colors appear unnecessarily conspicuous for the purpose they have to perform. In other words, it does not seem plausible that an animal already protected should need to be so conspicuous. If we stop for a moment to consider what an enormous amount of destruction must have occurred, according to Darwin’s theory, in order to bring this warning coloration to its supposed state of perfection, we may well hesitate before committing ourselves to such an extreme view.
That gaudy colors have appeared or been present in animals that are protected in other ways is not improbable, when we consider the rôle that color plays everywhere in nature. That the presence of such colors may, to a certain limited extent, protect its possessor may be admitted without in any degree supposing that natural selection has directed the evolution of such color, or that it has been acquired through a life and death struggle of the individuals of the species.
Sexual Dimorphism[[29]] and Trimorphism
[29]. This term is used here in the sense employed by Darwin. The same term is sometimes used for those cases in which the male departs very greatly from the female in form.
It has been found in a few species of animals and plants that two or more forms of one sex may exist, and here we find a condition that appears to be far more readily explained on the mutation theory than on any other. The most important cases, perhaps, are those in plants, but there are also similar cases known amongst animals, and these will be given first.
There is a North American butterfly, Papilio turnus, that appears under at least two forms. In the eastern United States the male has yellow wings with black stripes. There are two kinds of females, one of which resembles the male except that she has also an orange “eye-spot”; the other female is much blacker, and this variety is found particularly in the south and west. The species is dimorphic, therefore, mainly in the latter regions.
The cases of seasonal dimorphism offer somewhat similar illustrations. The European butterfly, Vanessa levana-prorsa, has a spring generation (levana) with a yellow and black pattern on the upper surface of the wings. The summer generation (prorsa) has black wings “with a broad white transverse band, and delicate yellow lines running parallel to the margins.” These two types are sharply separated, and their differences in color do not appear to be associated with any special protection that it confers on the bearer. These facts in regard to Vanessa seem to indicate that differences may arise that are perfectly well marked and sharply defined, which yet appear to be without any useful significance.
We meet with cases in which the same animal has at different times of year different colors, as seen in the summer and winter plumage of the ptarmigan. There is no direct evidence to show how this seasonable change has been brought about; but from the facts in regard to Vanessa we can see that it might have been at least possible for the white winter plumage, for instance, to have appeared without respect to any advantage it conferred on the animal, but after it had appeared it may have been to a certain degree useful to its possessor.
Fig. 5.—A, long-styled, and
B, short-styled, forms of Primula veris.
C, D, E, the three forms of the trimorphic flower of Lythrum salicaria, with petals and calyx removed on near side. (After Darwin.)
Amongst plants there are some very interesting cases of dimorphism and trimorphism in the structure of the flowers. Darwin has studied some of these cases with great care, and has made out some important points in regard to their powers of cross-fertilization.[[30]] The common European cowslip, Primula veris, var. officinalis, is found under two forms, Figure [5 A and B], which are about equally abundant. In one the style is long so that the stigma borne on its end comes to the top of the tube of the corolla. The stamens in this form stand about halfway up the tube. This is called the long-styled form. The other kind, known as the short-styled form, has a style only half as long as the tube of the corolla, and the stamens are attached around the upper end of the tube near its opening. In other words, the position of the end of the style (the stigma) and that of the stamens is exactly reversed in the two forms. The corolla is also somewhat differently shaped in the two forms, and the expanded part of the tube above the stamens is larger in the long-styled than in the short-styled form. Another difference is found in the stigma, which is globular in the long-styled, and depressed on its top in the short-styled, form. The papillæ on the former are twice as long as those on the short-styled form. The most important difference is found in the size of the pollen grains. These are larger in the long-styled form, being in the two cases in the proportion of 100 to 67. The shape of the grains is also different. Furthermore, the long-styled form tends to flower before the other kind, but the short-styled form produces more seeds. The ovules in the long-styled form, even when unfertilized, are considerably larger than those of the short-styled, and this, Darwin suggests, may be connected with the fact that fewer seeds are produced, since there is less room for them. The important point for our present consideration is that intermediate forms do not exist, although there are fluctuating variations about the two types. Moreover, the two kinds of flowers never appear on the same plant.
Darwin tried the effect of fertilizing the long-styled flowers with the pollen from the same flower or from other long-styled flowers. Unions of this sort he calls illegitimate, for reasons that will appear later. He also fertilized the long-styled flowers with pollen from short-styled forms. A union of this sort is called legitimate. Conversely, the short-styled forms were fertilized with their own pollen or with that from another short-styled form. This is also an illegitimate union. Short-styled forms fertilized with pollen from long-styled forms give again legitimate unions.
[30]. Many of the facts as to the occurrence of these cases were known before Darwin worked on them; but very little had been ascertained in regard to the sexual relation between the dimorphic and trimorphic forms, and it was here that Darwin obtained his most interesting results.
The outcome of these different crossings are most curious. In the table, page 364, the results of the four combinations are given. It will be seen at once that the legitimate unions give more capsules, and the seeds weigh more, than in the illegitimate unions.
The behavior of the offspring from seeds of legitimate and illegitimate origin is even more astonishing. Darwin found in Primula veris (the form just described) that the seeds from the short-styled form fertilized with pollen from the same form germinated so badly that he obtained only 14 plants, of which 9 were short-styled and 5 long-styled. The long-styled form fertilized with its own-styled pollen produced “in the first generation 3 long-styled plants. From their seed 53 long-styled grandchildren were produced; from their seed 4 long-styled great-grandchildren; from their seed 20 long-styled great-great-grandchildren; and lastly, from their seed 8 long-styled and 2 short-styled great-great-great-grandchildren.”
Nature of Union | Number of Flowers Fertilized | Number of Seed Capsules | Maximum of Seeds in any one Capsule | Minimum of Seeds in any one Capsule | Average No. of Seeds per Capsule |
|---|---|---|---|---|---|
| Long-styled form by pollen of short-styled form: Legitimate union. | 10 | 6 | 62 | 34 | 46.5 |
| Long-styled form by own-form pollen: Illegitimate union. | 20 | 4 | 49 | 2 | 27.7 |
| Short-styled form by pollen of long-styled form: Legitimate union. | 10 | 8 | 61 | 37 | 47.7 |
| Short-styled form by own-form pollen: Illegitimate union. | 17 | 3 | 19 | 6 | 12.1 |
| The two legitimate unions together. | 20 | 14 | 62 | 37 | 47.1 |
| The two illegitimate unions together. | 30 | 7 | 49 | 2 | 35.5 |
From other long-styled plants, fertilized with their own-form pollen, 72 plants were raised, which were made up of 68 long-styled and 4 short-styled. In all, 162 illegitimate unions of this sort produced 156 long-styled and 6 short-styled plants. It is evident from these results that the long-form pistils, fertilized with pollen from flowers of the same pistil-form (from other individuals as a rule), tend to produce the same form as their parents, although occasionally the other form. The fertility of these plants from an illegitimate union is found to be very low. Darwin observed that sometimes the male and female organs of these plants were in a very deteriorated condition. It is interesting to notice, in this connection, that in another species, Primula sinensis, illegitimate plants from long-styled parents were vigorous, but the flowers were small and more like the wild form. They were, however, perfectly fertile.
Illegitimate plants from short-styled parents were dwarfed in stature, and often had a weakly constitution. They were not very fertile inter se, and remarkably infertile when legitimately fertilized. This kind of result, where a difference in the power of mutual intercrossing exists between two forms, recalls in many ways the difference in the results of crossing of different species of animals and plants, especially those cases in which a cross can be made in one way more successfully than in the other.
The heterostyled trimorphic plants, of which Lythrum salicaria, Figure [5 C, D, E], may be taken as an example, are even more remarkable. There are three different kinds of flowers: in one the pistil is long and there is a medium and a short set of stamens; in another the pistil is of intermediate length and there is a long set and a short set of stamens; in the third kind the pistil is short, and there is a medium and a long set of stamens. There are possible only six sorts of legitimate unions between these three sets of flowers. No less than twelve kinds of illegitimate unions may occur. In regard to the difference in the sizes of the pollen grains, those from the long-styled form are the largest, from the mid-styled form next, and from the short-styled form the smallest. The extreme difference is as 100 to 60. “Nothing shows more clearly the extraordinary complexity of the reproductive system of this plant than the necessity of making eighteen distinct unions in order to ascertain the relative fertilizing power of the three forms.” Darwin tried the effect of each of these combinations, making 223 unions in all. The results are surprising. Comparing the outcome of the six legitimate unions with the twelve illegitimate ones, the following results were obtained:—
Nature of Union | Number of Flowers Fertilized | Number of Capsules Produced | Average No. of Seeds per Capsule | Average No. of Seeds per Flower Fertilized |
|---|---|---|---|---|
| The 6 legitimate unions | 75 | 56 | 96.29 | 71.89 |
| The 12 illegitimate unions | 146 | 36 | 44.72 | 11.03 |
This table shows that the fertility of the legitimate to that of the illegitimate is as 100 to 33, as judged by the flowers that produced capsules; and as 100 to 46 as judged by the average number of seeds per capsule. It is evident, therefore, that “it is only the pollen from the longest stamens that can fully fertilize the longest pistil; only that from the mid-length stamens, the mid-length pistil; and only that from the shortest stamens, the shortest pistil.”
Darwin tries to connect this fact with the visits of insects to the flowers. He says: “And now we can comprehend the meaning of the almost exact correspondence in length between the pistil in each form and of a set of six stamens in two of the other forms; for the stigma of each form is thus rubbed against that part of the insect’s body which becomes charged with the proper pollen.” A further conclusion that Darwin draws is “that the greater the inequality in length between the pistil and the set of stamens, the pollen of which is employed for its fertilization, by so much is the sterility the more increased.” Darwin also makes the following significant comment on the problem here involved: “The correspondence in length between the pistil in each form, and a set of stamens in the other two forms, is probably the direct result of adaptation, as it is of the highest service to the species by leading to full and legitimate fertilization.” He points out, on the other hand, that the increased sterility of the illegitimate unions, in proportion to the inequality in length between the pistil and the stamens employed, can be of no service at all. Neither can this relation have any connection with the facility for self-fertilization. “We are led, therefore, to conclude that the rule of increased sterility in accordance with increased inequality in length between the pistils and stamens is a purposeless result, incidental on those changes through which the species has passed in acquiring certain characters fitted to insure the legitimate fertilization of the three flowers.”
In regard to the plants that were raised from the seeds from legitimate and illegitimate unions, Darwin found in Lythrum that of twelve illegitimate unions two were completely barren, and nearly all showed lessened fertility; only one approached complete fertility. Darwin lays much emphasis on the close resemblance in the sterility of the illegitimate unions, and the sterility of different species when crossed. In both cases every degree of sterility is met with, “from very slightly lessened fertility to absolute barrenness.” The importance of this comparison cannot, I think, be overestimated, for, if admitted, it indicates clearly that the infertility between species cannot be used as a criterion of their distinctness, because here, in individuals belonging to the same species, we find sterility between pistils and stamens of different lengths. If, as I shall urge below, we must consider these different forms of Primula the results of a mutation, and not the outcome of selection as Darwin supposed, then this relation in regard to infertility becomes a point of great interest.
This brings us to the central point of our examination of these cases of dimorphism and trimorphism. How have these forms arisen? Darwin tries to account for them as follows: Since heterostyled plants occur in fourteen different families of plants, it is probable that this condition has been acquired independently in each family, and “that it can be acquired without any great difficulty.” The first step in the process he imagines to have been due to great variability in the length of the pistil and stamens, or of the pistil alone. Flowers in which there is a great deal of variation of this sort are known. “As most plants are occasionally cross-fertilized by the aid of insects, we may assume that this was the case with our supposed varying plant; but that it would have been beneficial to it to have been more regularly cross-fertilized.” “This would have been better accomplished if the stigma and the stamens stood at the same level; but as the stamens and pistil are supposed to have varied much in length, and to be still varying, it might well happen that they could be reduced much more easily through natural selection into two sets of different lengths in different individuals than all to the same length and level in all individuals.” By means of these assumptions, improbable as they may appear, Darwin tries to explain these cases of dimorphism. But when we attempt to apply the same argument to the trimorphic forms, it is manifestly absurd to pretend that three such sharply defined types could ever have been formed as the result of natural selection. But we have not even yet touched the chief difficulty, as Darwin himself points out. “The essential character of a heterostyled plant is that an individual of one form cannot fully fertilize, or be fertilized by, an individual of the same form, but only by one belonging to another form.” This result Darwin admits cannot be explained by the selection theory, for, as he says, “How can it be any advantage to a plant to be sterile with half of its brethren, that is, with the individuals belonging to the same form?” He concludes that this sterility between the individuals of the same form is an incidental and purposeless result. “Inner constitutional differences” between the individuals is the only suggestion that is offered to account for the phenomenon. In other words, it is clearly apparent that the attempt to apply the theory of selection has here broken down, and it is a fortunate circumstance that the Lamarckian theory cannot here be brought to the rescue, as it so often is in Darwin’s writings, when the theory of natural selection fails to give a sufficient explanation.
On the other hand, this is one of the cases that seem to fit in excellently with the mutation theory, for if these two forms of the primrose should appear, as mutations, and if, as is the case, they do not blend when crossed, but are equally inherited, they would both continue to exist as we find them to-day. Whether the similar forms were infertile with each other would be determined at the outset by the nature of the individual variation, and if, despite this obvious disadvantage, the forms could still continue to propagate themselves, the new dimorphic form would remain in existence. Darwin cannot explain the origin of dimorphic forms and trimorphic forms unless he can show that there is some advantage in having two forms, and as we have seen, he fails completely to show that there is an advantage. On the other hand, the result might have been reached on the mutation theory, even if the dimorphic and trimorphic forms were placed at a greater disadvantage than were the parent forms. In such a case fewer individuals might appear, or find a foothold; but as long as the race could be kept up the new forms would remain in existence. Thus, while no attempt is made to explain what has always been, and may possibly long remain, inexplicable to us, namely, the origin of the new form itself, yet granting that such new forms may sometimes appear spontaneously, they may be able to establish themselves, regardless of whether they are a little more or a little less well adapted to the environment than were their parent forms. If it should appear that the question is begged by the assumption that mutations such as these may appear (at one step or by a series of steps is immaterial), it should not be forgotten that the whole Darwinian theory itself also rests on the spontaneous appearance of fluctuating variations, whose origin it does not pretend to explain. In this respect both theories are on the same footing, but where the Darwinian theory meets with difficulties at every turn by assuming that new forms are built up through the action of selection, the mutation theory escapes most of these difficulties, because it applies no such rigid test as that of selection to account for the presence of new forms.