THE SHAPES OF BIRDS
THE enormous and sudden advance made by zoological science in the latter half of the nineteenth century has been followed by a reaction. During the last ten or twelve years that particular branch of knowledge has made comparatively little progress. Darwin and Wallace completely revolutionized biology. They shed the light of the highest genius on the darkness which had hitherto brooded over the study of life. Their researches gave an enormous impetus to natural science. Nor were these the only stimuli. The theory of natural selection met at first with very bitter opposition on all sides. This opposition stirred up the Darwinians to new exertions.
Unfortunately the opposition was very shortlived. The triumph of the theory of natural selection was as speedy as it was complete. It would, I believe, have been more profitable to biological science had the conflict been of longer duration. Natural selection has won all along the line. It has proved itself able to explain a large number of phenomena, it has overcome a multitude of difficulties. Facts which were at one time urged against it are now held to be among the most powerful arguments in its favour. It is to-day almost universally accepted as a solution of all biological problems. It has come to be regarded with almost superstitious reverence as the master-key which is able to open the doors of all the passages which lead to the secret chambers of Nature. So great is our confidence in the powers of this master-key that we have even neglected to put it to the test in some cases. It has succeeded in very many instances, we therefore assume that it must be successful in all. It has unlocked the main doors, hence we deem it unnecessary to try it with smaller ones.
In other words, zoological science is in danger of stagnation. I admit that much useful work is being accomplished. Never before were so many workers in the field. A mass of new facts is accumulating. Daily, fresh contributions are added to our zoological knowledge. But each worker restricts himself to one small portion of the field, so that the main theory has made but little progress.
It is time that there was a fresh stocktaking; that the new facts discovered were co-ordinated, and their relations to one another and to the main theory studied. At present the tendency is to attribute almost supernatural powers to natural selection, to believe that it is the key to every biological problem.
If we ask why an animal is of such-and-such a colour, we are told natural selection has given the creature its colour as being that best suited to its needs. If we say that we fail to see how that particular colour is more useful to the animal than every other, we are told that as soon as we learn all the habits of the creature in question we shall see how perfectly its colour is adapted to its mode of life. This may be so. Nevertheless this kind of argument is not scientific. It tends to stifle inquiry, which is the true spirit of science.
The fact is that natural selection is a horse ridden to death. It is indisputably a most important factor in organic evolution, but are we justified in regarding it as the only factor ? It is unable, I think, to explain many natural phenomena. One of these is the varying shapes of nearly allied animals.
Certain it is that the general form of a class of organisms is determined by natural selection, but are the thousand and one shapes seen among closely related creatures all to be explained by saying that were these of any other form they would perish in the struggle for existence ?
Birds afford a striking example of the many shapes which may be assumed by creatures of very similar habits. I recently visited the Nilgiris, and spent many hours in a wood which might appropriately be called " The Flycatchers' Wood." No fewer than five species of that family are common in the wood of which the area is less than 5000 square yards. All these species have very similar habits.
To enumerate them. There is first the white-browed fantail flycatcher (Rhipidura albifrontata), a bird too well known to need detailed description. It will suffice that its chief characteristic is the tail, which it continually spreads out into a fan. This appendage is about three and a half inches long, that is to say, equal in length to the rest of the bird. Next comes the black and orange flycatcher (Ochromela nigrirufa), which looks for all the world like a robin. Its tail is only two inches long, while the body is three. Then there is the greyheaded flycatcher (Culicicapa ceylonensis). This, too, is a squat-figured little bird. The Nilgiri blue flycatcher (Stoparola albicaudata) next demands notice. In shape it differs from all the three birds mentioned above. Its tail is relatively short, and its body slim and elongated by comparison with the grey-headed and black and orange species. Moreover, it is sexually dimorphic. The male is indigo blue, while the female is brownish. There remains Tickell's blue flycatcher (Cyornis tickelli). This is a beautiful little bird, differing in shape from the birds already mentioned to such an extent that, quite apart from its distinctive plumage, it would be impossible to confound it with any of them. I did not see the paradise flycatcher (Terpsiphone paradisi), but the bird is found in the Nilgiris and probably visits the wood in question. The male of this species, when he comes of age, has a tail sixteen inches long; that is to say, four times the length of his body, while the tail of the hen bird makes up but half of her total length.
These birds, which display considerable variety as regards shape, have very similar habits. They all feed on insects, which, to quote Mr. Oates, they either catch on the wing, starting from a perch to which they usually return several times, or by running with the aid of their wings along the limbs of trees. I believe that of those the fantail species alone runs along the branches of trees. Mr. Oates adds, " they seldom or never descend to the ground."
This statement is not strictly true. I have repeatedly seen the fantail, the grey, and the black and orange species on the ground. But the point I desire to emphasise is that their methods of obtaining food are all very much the same. Were all the species of the same colour and shape, I think few observers would be able to distinguish one species from another, merely by watching their methods of securing food. Their varied nesting habits would, of course, serve to distinguish them.
Here, then, we have five species of birds, living side by side, under similar conditions and eating the same description of food, obtained by like methods, yet arrayed in totally different plumage and of varying form.
Passing over the differences in colouration, let us confine ourselves to configuration. Why are these birds not all of the same shape? They are related to one another; all are descendants of a common ancestor, and, as we have seen, their methods of obtaining food are not marked by any considerable differences; why, then, are they not all of one shape -: the shape best suited to flycatching birds ?
I do not think for a moment that it is possible successfully to maintain that the shape of each particular species is so important to it that, were the bird of any other shape, it must perish in the struggle for existence. The paradise flycatcher disproves such an hypothesis. The male and female differ considerably in form, yet both are equally successful in obtaining food, and both secure it in the same manner. Moreover, the young male has a tail four inches in length, but, later on, he grows one sixteen inches long, yet he continues to obtain food in the same manner. Thus a difference of twelve inches in the length of his tail does not appreciably affect his ability to find food.
Even if we could demonstrate that each species takes the shape best suited to its mode of life, if we could prove, for example, that the Nilgiri blue flycatcher would be greatly handicapped in the search for food were his shape that of the grey-headed flycatcher, this would not be sufficient. If natural selection alone is responsible for the shape of an organism, we must prove that every step in the transition from the common ancestral form to that of the present species was a distinct gain to the species. This point is often lost sight of by those who invoke the aid of natural selection to explain every zoological difficulty. It seems to me that the great diversity in shape exhibited by birds having similar habits merely shows that there are several equally good methods of accomplishing an object.
If Nature desires to call into existence a number of flycatching birds, she is not obliged to cast all in exactly the same mould; she is able to create many different forms of organism, all well adapted to the work before them. The general shape is of course determined by natural selection, especially in the case of highly specialized birds, such as woodpeckers, kingfishers, and swifts. But, even in such cases, considerable diversity of form is permitted. The less specialized the habits of a bird are, the greater is the latitude as regards shape allowed to it.
The shape of organisms is due to the action of a large number of forces, of most of which we are totally ignorant. Natural selection does not interfere unless the variation in shape tends to benefit or injuriously affect the possessor. In the former case, the beneficial shape tends to be perpetuated and to cause the species to spread at the expense of other less-favoured ones. In the latter case the injurious variation leads to the extermination of the creatures in which it appears.
Natural selection, like the stone walls of a labyrinth of lanes, marks certain limits within which variations as regards shape may persist. So long as the variations are such as do not affect the mobility of a species, its ability to obtain food, or its relationship to its environment, natural selection does not in any way interfere.
The causes which have produced this diversity of shape among allied species and genera have yet to be discovered. We are not at present in a position to say why some birds are large and others small, why some are slim and others stout, why some have pointed wings and others round ones, why some have broad heads and others narrow ones. It is useless to pretend that natural selection explains all these phenomena. It is better to be honest and frankly admit our ignorance.