Contributions to the Theory of Natural Selection Part 13

The female bird, while sitting on her eggs in an uncovered nest, is much exposed to the attacks of enemies, and any modification of colour which rendered her more conspicuous would often lead to her destruction and that of her offspring. All variations of colour in this direction in the female, would therefore sooner or later be eliminated, while such modifications as rendered her inconspicuous, by a.s.similating her to surrounding objects, as the earth or the foliage, would, on the whole, survive the longest, and thus lead to the attainment of those brown or green and inconspicuous tints, which form the colouring (of the upper surface at least), of the vast majority of female birds which sit upon open nests.

This does not imply, as some have thought, that all female birds were once as brilliant as the males. The change has been a very gradual one, generally dating from the origin of genera or of larger groups, but there can be no doubt that the remote ancestry of birds having great s.e.xual differences of colour, were nearly or quite alike, sometimes (perhaps in most cases) more nearly resembling the female, but occasionally perhaps being nearer what the male is now. The young birds (which usually resemble the females) will probably give some idea of this ancestral type, and it is well known that the young of allied species and of different s.e.xes are often undistinguishable.

_Colour more variable than Structure or Habits, and therefore the Character which has generally been Modified._

At the commencement of this essay, I have endeavoured to prove, that the characteristic differences and the essential features of birds' nests, are dependent on the structure of the species and upon the present and past conditions of their existence. Both these factors are more important and less variable than colour; and we must therefore conclude that in most cases the mode of nidification (dependent on structure and environment) has been the cause, and not the effect, of the similarity or differences of the s.e.xes as regards colour. When the confirmed habit of a group of birds, was to build their nests in holes of trees like the toucans, or in holes in the ground like the kingfishers, the protection the female thus obtained, during the important and dangerous time of incubation, placed the two s.e.xes on an equality as regards exposure to attack, and allowed "s.e.xual selection," or any other cause, to act unchecked in the development of gay colours and conspicuous markings in both s.e.xes.

When, on the other hand (as in the Tanagers and Flycatchers), the habit of the whole group was to build open cup-shaped nests in more or less exposed situations, the production of colour and marking in the female, by whatever cause, was continually checked by its rendering her too conspicuous, while in the male it had free play, and developed in him the most gorgeous hues. This, however, was not perhaps universally the case; for where there was more than usual intelligence and capacity for change of habits, the danger the female was exposed to by a partial brightness of colour or marking might lead to the construction of a concealed or covered nest, as in the case of the t.i.ts and Hangnests.

When this occurred, a special protection to the female would be no longer necessary; so that the acquisition of colour and the modification of the nest, might in some cases act and react on each other and attain their full development together.

_Exceptional Cases confirmatory of the above Explanation._

There exist a few very curious and anomalous facts in the natural history of birds, which fortunately serve as crucial tests of the truth of this mode of explaining the inequalities of s.e.xual colouration. It has been long known, that in some species the males either a.s.sisted in, or wholly performed, the act of incubation. It has also been often noticed, that in certain birds the usual s.e.xual differences were reversed, the male being the more plainly coloured, the female more gay and often larger. I am not, however, aware that these two anomalies had ever been supposed to stand to each other in the relation of cause and effect, till I adduced them in support of my views of the general theory of protective adaptation. Yet it is undoubtedly the fact, that in the best known cases in which the female bird is more conspicuously coloured than the male, it is either positively ascertained that the latter performs the duties of incubation, or there are good reasons for believing such to be the case. The most satisfactory example is that of the Gray Phalarope (Phalaropus fulicarius), the s.e.xes of which are alike in winter, while in summer the female instead of the male takes on a gay and conspicuous nuptial plumage; but the male performs the duties of incubation, sitting upon the eggs, which are laid upon the bare ground.

In the Dotterell (Eudromias morinellus) the female is larger and more brightly coloured than the male; and here, also, it is almost certain that the latter sits upon the eggs. The Turnices of India also, have the female larger and often more brightly coloured; and Mr. Jerdon states, in his "Birds of India," that the natives report, that, during the breeding season, the females desert their eggs and a.s.sociate in flocks, while the males are employed in hatching the eggs. In the few other cases in which the females are more brightly coloured, the habits are not accurately known. The case of the Ostriches and Emeus will occur to many as a difficulty, for here the male incubates, but is not less conspicuous than the female; but there are two reasons why the case does not apply;--the birds are too large to derive any safety from concealment, from enemies which would devour the eggs they can defend themselves by force, while to escape from their personal foes they trust to speed.

We find, therefore, that a very large ma.s.s of facts relating to the s.e.xual colouration and the mode of nidification of birds, including some of the most extraordinary anomalies to be found in their natural history, can be shown to have an interdependent relation to each other, on the simple principle of the need of greater protection to that parent which performs the duties of incubation. Considering the very imperfect knowledge we possess of the habits of most extra-European birds, the exceptions to the prevalent rule are few, and generally occur in isolated species or in small groups; while several apparent exceptions can be shown to be really confirmations of the law.

_Real or apparent Exceptions to the Law stated at page 240._

The only marked exceptions I have been able to discover are the following:--

1. King crows (Dicrourus). These birds are of a glossy black colour with long forked tails. The s.e.xes present no difference, and they build open nests. This apparent exception may probably be accounted for by the fact that these birds do not need the protection of a less conspicuous colour. They are very pugnacious, and often attack and drive away crows, hawks, and kites; and as they are semi-gregarious in their habits, the females are not likely to be attacked while incubating.

2. Orioles (Oriolidae). The true orioles are very gay birds; the s.e.xes are, in many Eastern species, either nearly or quite alike, and the nests are open. This is one of the most serious exceptions, but it is one that to some extent proves the rule; for in this case it has been noticed, that the parent birds display excessive care and solicitude in concealing the nest among thick foliage, and in protecting their offspring by incessant and anxious watching. This indicates that the want of protection consequent on the bright colour of the female makes itself felt, and is obviated by an increased development of the mental faculties.

3. Ground thrushes (Pittidae). These elegant and brilliantly-coloured birds are generally alike in both s.e.xes, and build an open nest. It is curious, however, that this is only an apparent exception, for almost all the bright colours are on the under surface, the back being usually olive green or brown, and the head black, with brown or whitish stripes, all which colours would harmonize with the foliage, sticks, and roots which surround the nest, built on or near the ground, and thus serve as a protection to the female bird.

4. Grallina Australis. This Australian bird is of strongly contrasted black and white colours. The s.e.xes are exactly alike, and it builds an open clay nest in an exposed situation on a tree. This appears to be a most striking exception, but I am by no means sure that it is so. We require to know what tree it usually builds on, the colour of the bark or of the lichens that grow upon it, the tints of the ground, or of other surrounding objects, before we can say that the bird, when sitting on its nest, is really conspicuous. It has been remarked that small patches of white and black blend at a short distance to form grey, one of the commonest tints of natural objects.

5. Sunbirds (Nectarineidae). In these beautiful little birds the males only are adorned with brilliant colours, the females being quite plain, yet they build covered nests in all the cases in which the nidification is known. This is a negative rather than a positive exception to the rule, since there may be other causes besides the need for protection, which prevent the female acquiring the gay colours of her mate, and there is one curious circ.u.mstance which tends to elucidate it. The male of Leptocoma zeylanica is said to a.s.sist in incubation. It is possible, therefore, that the group may originally have used open nests, and some change of conditions, leading the male bird to sit, may have been followed by the adoption of a domed nest. This is, however, the most serious exception I have yet found to the general rule.

6. Superb warblers (Maluridae). The males of these little birds are adorned with the most gorgeous colours, while the females are very plain, yet they make domed nests. It is to be observed, however, that the male plumage is nuptial merely, and is retained for a very short time; the rest of the year both s.e.xes are plain alike. It is probable, therefore, that the domed nest is for the protection of these delicate little birds against the rain, and that there is some unknown cause which has led to the development of colour in the males only.

There is one other case which at first sight looks like an exception, but which is far from being one in reality, and deserves to be mentioned. In the beautiful Waxwing, (Bombycilla garrula,) the s.e.xes are very nearly alike, and the elegant red wax tips to the wing-feathers are nearly, and sometimes quite, as conspicuous in the female as in the male. Yet it builds an open nest, and a person looking at the bird would say it ought according to my theory to cover its nest. But it is, in reality, as completely protected by its colouration as the most plainly coloured bird that flies. It breeds only in very high lat.i.tudes, and the nest, placed in fir-trees, is formed chiefly of lichens. Now the delicate gray and ashy and purplish hues of the head and back, together with the yellow of the wings and tail, are tints that exactly harmonize with the colours of various species of lichens, while the brilliant red wax tips exactly represent the crimson fructification of the common lichen, Cladonia coccifera. When sitting on its nest, therefore, the female bird will exhibit no colours that are not common to the materials of which it is constructed; and the several tints are distributed in about the same proportions as they occur in nature. At a short distance the bird would be indistinguishable from the nest it is sitting on, or from a natural clump of lichens, and will thus be completely protected.

I think I have now noticed all exceptions of any importance to the law of dependence of s.e.xual colour on nidification. It will be seen that they are very few in number, compared with those which support the generalization; and in several cases there are circ.u.mstances in the habits or structure of the species that sufficiently explain them. It is remarkable also that I have found scarcely any _positive_ exceptions, that is, cases of very brilliant or conspicuous female birds in which the nest was not concealed. Much less can there be shown any group of birds, in which the females are all of decidedly conspicuous colours on the upper surface, and yet sit in open nests. The many cases in which birds of dull colours in both s.e.xes make domed or concealed nests, do not, of course, affect this theory one way or the other; since its purpose is only to account for the fact, that brilliant females of brilliant males are _always_ found to have covered or hidden nests, while obscure females of brilliant males _almost always_ have open and exposed nests. The fact that all cla.s.ses of nests occur with dull coloured birds in both s.e.xes merely shows, as I have strongly maintained, that in most cases the character of the nest determines the colouration of the female, and not _vice versa_.

If the views here advocated are correct, as to the various influences that have determined the specialities of every bird's nest, and the general colouration of female birds, with their action and reaction on each other, we can hardly expect to find evidence more complete than that here set forth. Nature is such a tangled web of complex relations, that a series of correspondences running through hundreds of species, genera, and families, in every part of the system, can hardly fail to indicate a true casual connexion; and when, of the two factors in the problem, one can be shown to be dependent on the most deeply seated and the most stable facts of structure and conditions of life, while the other is a character universally admitted to be superficial and easily modified, there can be little doubt as to which is cause and which effect.

_Various modes of Protection of Animals._

But the explanation of the phenomenon here attempted does not rest alone on the facts I have been able now to adduce. In the essay on "Mimicry,"

it is shown how important a part the necessity for protection has played, in determining the external form and colouration, and sometimes even the internal structure of animals.

As ill.u.s.trating this latter point, I may refer to the remarkable hooked, branched, or star-like spiculae in many sponges, which are believed to have the function chiefly, of rendering them unpalatable to other creatures. The Holothuridae or sea-cuc.u.mbers possess a similar protection, many of them having anchor-shaped spicules embedded in their skin, as the Synapta; while others (Cuviera squamata) are covered with a hard calcareous pavement. Many of these are of a bright red or purple colour, and are very conspicuous, while the allied Trepang, or Beche-de-mer (Holothuria edulis), which is not armed with any such defensive weapons, is of a dull sand-or mud-colour, so as hardly to be distinguished from the sea bed on which it reposes. Many of the smaller marine animals are protected by their almost invisible transparency, while those that are most brightly coloured will be often found to have a special protection, either in stinging tentacles like Physalia, or in a hard calcareous crust, as in the star fishes.

_Females of some Groups require and obtain more Protection than the Males._

In the struggle for existence incessantly going on, protection or concealment is one of the most general and most effectual means of maintaining life; and it is by modifications of colour that this protection can be most readily obtained, since no other character is subject to such numerous and rapid variations. The case I have now endeavoured to ill.u.s.trate is exactly a.n.a.logous to what occurs among b.u.t.terflies. As a general rule, the female b.u.t.terfly is of dull and inconspicuous colours, even when the male is most gorgeously arrayed; but when the species is protected from attack by a disagreeable odour, as in the Heliconidae, Danaidae and Acroeidae, both s.e.xes display the same or equally brilliant hues. Among the species which gain a protection by imitating these, the very weak and slow-flying Leptalides resemble them in both s.e.xes, because both s.e.xes alike require protection, while in the more active and strong-winged genera--Papilio, Pieris, and Diadema--it is generally the females only that mimic the protected groups, and in doing so often become actually more gay and more conspicuous than the males, thus reversing the usual and in fact almost universal characters of the s.e.xes. So, in the wonderful Eastern leaf-insects of the genus Phyllium, it is the female only that so marvellously imitates a green leaf; and in all these cases the difference can be traced to the greater need of protection for the female, on whose continued existence, while depositing her eggs, the safety of the race depends. In Mammalia and in reptiles, however brilliant the colours may be, there is rarely any difference between that of the s.e.xes, because the female is not necessarily more exposed to attack than the male. It may, I think, be looked upon as a confirmation of this view, that no single case is known either in the above-named genera--Papilio, Pieris, and Diadema--or in any other b.u.t.terfly, of a male _alone_, mimicking one of the Danaidae or Heliconidae. Yet the necessary colour is far more abundant in the males, and variations always seem ready for any useful purpose. This seems to depend on the general law, that each species and each s.e.x can only be modified just as far as is absolutely necessary for it to maintain itself in the struggle for existence, not a step further. A male insect by its structure and habits is less exposed to danger, and also requires less protection than the female. It cannot, therefore, alone acquire any further protection through the agency of natural selection. But the female requires some extra protection, to balance the greater danger to which she is exposed, and her greater importance to the existence of the species; and this she always acquires, in one way or another, through the action of natural selection.

In his "Origin of Species," fourth edition, p. 241, Mr. Darwin recognises the necessity for protection as sometimes being a cause of the obscure colours of female birds; but he does not seem to consider it so very important an agent in modifying colour as I am disposed to do.

In the same paragraph (p. 240), he alludes to the fact of female birds and b.u.t.terflies being sometimes very plain, sometimes as gay as the males; but, apparently, considers this mainly due to peculiar laws of inheritance, which sometimes continue acquired colour in the line of one s.e.x only, sometimes in both. Without denying the action of such a law (which Mr. Darwin informs me he has facts to support), I impute the difference, in the great majority of cases, to the greater or less need of protection in the female s.e.x in these groups of animals.

This need was seen to exist a century ago by the Hon. Daines Barrington, who, in the article already quoted (see p. 220), after alluding to the fact that singing birds are all small, and suggesting (but I think erroneously) that this may have arisen from the difficulty larger birds would have in concealing themselves if they called the attention of their enemies by loud notes, goes on thus:--"I should rather conceive it is for the same reason no hen bird sings, because this talent would be still more dangerous during incubation, which _may possibly also account for the inferiority in point of plumage_." This is a curious antic.i.p.ation of the main idea on which this essay is founded. It has been unnoticed for near a century, and my attention was only recently called to it by Mr. Darwin himself.

_Conclusion._

To some persons it will perhaps appear, that the causes to which I impute so much of the external aspect of nature are too simple, too insignificant, and too unimportant for such a mighty work. But I would ask them to consider, that the great object of all the peculiarities of animal structure is to preserve the life of the individual, and to maintain the existence of the species. Colour has. .h.i.therto been too often looked upon as something advent.i.tious and superficial, something given to an animal not to be useful to itself, but solely to gratify man or even superior beings--to add to the beauty and ideal harmony of nature. If this were the case, then, it is evident that the colours of organised beings would be an exception to most other natural phenomena.

They would not be the product of general laws, or determined by ever-changing external conditions; and we must give up all enquiry into their origin and causes, since (by the hypothesis) they are dependent on a Will whose motives must ever be unknown to us. But, strange to say, no sooner do we begin to examine and cla.s.sify the colours of natural objects, than we find that they are intimately related to a variety of other phenomena, and are, like them, strictly subordinated to general laws. I have here attempted to elucidate some of these laws in the case of birds, and have shown how the mode of nidification has affected the colouring of the female s.e.x in this group. I have before shown to how great an extent, and in how many ways, the need of protection has determined the colours of insects, and of some groups of reptiles and mammalia, and I would now call particular attention to the fact that the gay tints of flowers, so long supposed to be a convincing proof that colour has been bestowed for other purposes than the good of its possessor, have been shown by Mr. Darwin to follow the same great law of utility. Flowers do not often need protection, but very often require the aid of insects to fertilize them, and maintain their reproductive powers in the greatest vigour. Their gay colours attract insects, as do also their sweet odours and honeyed secretions; and that this is the main function of colour in flowers is shown by the striking fact, that those flowers which can be perfectly fertilized by the wind, and do not need the aid of insects, _rarely or never have gaily-coloured flowers_.

This wide extension of the general principle of utility to the colours of such varied groups, both in the animal and vegetable kingdoms, compels us to acknowledge that the "reign of law" has been fairly traced into this stronghold of the advocates of special creation. And to those who oppose the explanation I have given of the facts adduced in this essay, I would again respectfully urge that they must grapple with the whole of the facts, not one or two of them only. It will be admitted that, on the theory of evolution and natural selection, a wide range of facts with regard to colour in nature have been co-ordinated and explained. Until at least an equally wide range of facts can be shown to be in harmony with any other theory, we can hardly be expected to abandon that which has already done such good service, and which has led to the discovery of so many interesting and unexpected harmonies among the most common (but hitherto most neglected and least understood), of the phenomena presented by organised beings.

VIII.

CREATION BY LAW.

Among the various criticisms that have appeared on Mr. Darwin's celebrated "Origin of Species," there is, perhaps, none that will appeal to so large a number of well educated and intelligent persons, as that contained in the Duke of Argyll's "Reign of Law." The n.o.ble author represents the feelings and expresses the ideas of that large cla.s.s, who take a keen interest in the progress of Science in general, and especially that of Natural History, but have never themselves studied nature in detail, or acquired that personal knowledge of the structure of closely allied forms,--the wonderful gradations from species to species and from group to group, and the infinite variety of the phenomena of "variation" in organic beings,--which are absolutely necessary for a full appreciation of the facts and reasonings contained in Mr. Darwin's great work.

Nearly half of the Duke's book is devoted to an exposition of his idea of "Creation by Law," and he expresses so clearly what are his difficulties and objections as regards the theory of "Natural Selection," that I think it advisable that they should be fairly answered, and that his own views should be shown to lead to conclusions, as hard to accept as any which he imputes to Mr. Darwin.

The point on which the Duke of Argyll lays most stress, is, that proofs of Mind everywhere meet us in Nature, and are more especially manifest wherever we find "contrivance" or "beauty." He maintains that this indicates the constant supervision and direct interference of the Creator, and cannot possibly be explained by the una.s.sisted action of any combination of laws. Now, Mr. Darwin's work has for its main object, to show, that all the phenomena of living things,--all their wonderful organs and complicated structures, their infinite variety of form, size, and colour, their intricate and involved relations to each other,--may have been produced by the action of a few general laws of the simplest kind, laws which are in most cases mere statements of admitted facts.

The chief of these laws or facts are the following:--

1. _The Law of Multiplication in Geometrical Progression._--All organized beings have enormous powers of multiplication. Even man, who increases slower than all other animals, could under the most favourable circ.u.mstances double his numbers every fifteen years, or a hundred-fold in a century. Many animals and plants could increase their numbers from ten to a thousand-fold every year.

2. _The Law of Limited Populations._--The number of living individuals of each species in any country, or in the whole globe, is practically stationary; whence it follows that the whole of this enormous increase must die off almost as fast as produced, except only those individuals for whom room is made by the death of parents. As a simple but striking example, take an oak forest. Every oak will drop annually thousands or millions of acorns, but till an old tree falls, not one of these millions can grow up into an oak. They must die at various stages of growth.

3. _The Law of Heredity, or Likeness of Offspring to their Parents._--This is a universal, but not an absolute law. All creatures resemble their parents in a high degree, and in the majority of cases very accurately; so that even individual peculiarities, of whatever kind, in the parents, are almost always transmitted to some of the offspring.

4. _The Law of Variation._--This is fully expressed by the lines:--

"No being on this earthly ball, Is like another, all in all."

Offspring resemble their parents very much, but not wholly--each being possesses its individuality. This "variation" itself varies in amount, but it is always present, not only in the whole being, but in every part of every being. Every organ, every character, every feeling is individual; that is to say, _varies_ from the same organ, character, or feeling in every other individual.

5. _The Law of unceasing Change of Physical Conditions upon the Surface of the Earth._--Geology shows us that this change has always gone on in times past, and we also know that it is now everywhere going on.

6. _The Equilibrium or Harmony of Nature._--When a species is well adapted to the conditions which environ it, it flourishes; when imperfectly adapted it decays; when ill-adapted it becomes extinct. If _all_ the conditions which determine an organism's well-being are taken into consideration, this statement can hardly be disputed.