An excerpt from a typical, cheap then and even cheaper now ($5) little pocket book written when evolution was called 'evolution':"
What is a bird? The text-books tell us that it is a warm-blooded, egg-laying, feathered biped, whose fore limbs take the form of wings, and whose jaws are encased in a horny sheath to form a beak.
Such a definition will suffice for most of us, but not for all. A few there be who would know more; who are curious concerning origins, and will not be satisfied by any such cut and dried definition; for a definition is not an explanation . . .
From the evidence which we have collected so far, we gather that what we now know as a bird has reached its present form by a slow process of change from some other, which was probably more or less, like what we designate to-day a reptile, e.g. lizard. We say more or less, advisedly; for the lizard, in its turn, has an equally mysterious origin. One reason then for thinking the bird in some way reptilian is because it shares many things in common with this group, things which occur nowhere else outside, things that are shared by both, probably by virtue of descent from a common stock.
The bird has risen in the world so as to rank, by common consent of men of science—who fill the part of Nature's Herald's Office—higher than the reptiles. These represent, to-day, its poor relations.
This gradual change of form, from a more or less like and uniform beginning, the ancestral stock, and the division of this stock into two great classes reptiles and birds, we call evolution. Exactly how this evolution has come about even those best qualified to speak do not entirely agree.
The hypothesis most generally in favour at the present day is that of Darwin and Wallace, and known as 'Natural Selection.' According to this hypothesis the interaction of living organisms, one upon another, involves a 'struggle for existence' in which the 'fittest' survive, and the unfit are eliminated—the unfit representing those who cannot respond, or adapt themselves to changes in their surroundings or environment.
Although much abused, and generally misunderstood and misrepresented, this is nevertheless for the most part admitted to be the best and most satisfactory of any hypothesis yet offered us.
What answer will this give us to the question, How did the pigeons, ducks, geese, and songbirds, come to be?
Well, it would first of all draw our attention to the fact that 'like begets like'. That though the pigeon, the lizard and the snake all lay eggs, yet the pigeon's egg will never bring forth anything but another pigeon.
It will next point out a much less familiar fact that
'No being on this earthly ball
Is like another, all in all.'
Now that is a most important observation. Just as all the children of a family differ one from another, so do all the birds of the same nest, though, perhaps, not so markedly to our eyes. Let us follow this up.
The children of this family intermarry with those of some other, and their children yet again with another, and so on. If a census was taken of the population of, say a village, and careful measurements and observations taken as to the height, sight, and hearing, colour of the hair and eyes, freedom from disease, and so on, we should find that half of the whole number would fall below, and half would rise above a common mean. The mean, of course, would be different for each test. Thus a large number of those who were included as above the average in height might find themselves bracketed as below the average in power of endurance, and so on.
In this tendency to vary we have the raw material used in Nature's workshop in the manufacture of new types.
Man long ago seized upon this fact and turned it to his own advantage, as witness our garden flowers and vegetables, and our domestic cattle. These have only reached their present form by slow degrees, each stage improving upon the last. The result of such a series of progressive changes must be to remove slowly from, and obliterate in, the individuals concerned, the traces of their original likeness.
In the case of domestic animals we can constantly compare the latest variety with the original stock from which it was derived. For instance, all the domestic pigeons are descended from our common rock dove. Selective breeding—that is to say, breeding with a definite end in view—has resulted in numerous varieties so unlike one another, and the original, that did they exist in a wild state, we should regard each as a separate 'species'. When it was desired, for instance, to obtain birds with enormous fleshy lobes or wattles round the beak and eyes, those young, in chish this feature was most developed were selected to breed from, the rest went to supply the table. The offspring of the selected were similarly treated, in this way the character grew more and more marked till the present somewhat unsightly forms came into being . . . A similar process of selective breeding seems to be going on around us, amongst wild animals and wild plants. It has given us the hare and rabbit, wolf and fox, lion and tiger, and so on . . .
We find certain birds fitted apparently to live only upon certain spots on the earth's surface or upon certain food. We say these are highly specialised
. . . .
Specialisation may be carried to such a degree of perfection that any considerable change in the environment of an animal may cause its extinction . . . Let us take (an illustration) from ourselves. The labourer corresponds to what we call a generalised type. He can earn his living in many ways; the watchmaker, or woodcarver, or artist, only in one. If the latter are prevented from carrying on their trade, starvation and death stare them in the face—they are too specialised to turn to some new mode of earning a livelihood. "
W.P. Pycraft, The Story of Bird Life, George Newnes Limited, London, 1900
excerpt from Chapter I,"What is a bird?—Its form and structure"