Evolution and Adaptation

by Thomas Hunt Morgan

Darwin thinks that a change in climate alone is not one of the potent causes of variability, because the native country of a plant, where it has been longest cultivated, is where it has oftenest given rise to the greatest number of varieties. He thinks it also doubtful that a change in food is an important source of variability, since the domestic pigeon has varied more than any other species of fowl, yet the food has been always nearly the same. This is also true for cattle and sheep, whose food is probably much less varied in kind than in the wild species.

Another point of interest is raised by Darwin. He thinks, as do others also, that the influence of a change in the conditions is cumulative, in the sense that it may not appear until the species has been subjected to it for several generations. Darwin states that universal experience shows that when new plants are first introduced into gardens they do not vary, but after several generations they will begin to vary to a greater or less extent. In a few cases, as in that of the dahlia, the zinnia, the Swan River daisy, and the Scotch rose, it is known that the new variations only appeared after a time. The following statement by Salter is then quoted, “Every one knows that the chief difficulty is in breaking through the original form and color of the species, and every one will be on the lookout for any natural sport, either from seed or branch; that being once obtained, however trifling the change may be, the result depends on himself.” Jonghe is also quoted to the effect that “there is another principle, namely, that the more a type has entered into a state of variation, the greater is the tendency to continue doing so, and the more it has varied from the original type, the more is it disposed to vary still further.” Darwin also quotes with approval the opinion of the most celebrated horticulturist of France, Vilmorin, who maintained that “when any particular variation is desired, the first step is to get the plant to vary in any manner whatever, and to go on selecting the most variable individuals, even though they vary in the wrong direction; for the fixed character of the species being once broken, the desired variation will sooner or later appear.”

Darwin also cites a few cases where animals have changed quite quickly when brought under domestication. Turkeys raised from the eggs of wild species lose their metallic tints, and become spotted with white in the third generation. Wild ducks lose their true plumage after a few generations. “The white collar around the neck of the mallard becomes much broader and more irregular, and white feathers appear in the duckling’s wings. They increase also in size of body.” In these cases it appears that several generations were necessary in order to bring about a marked change in the original type, but the Australian dingoes, bred in the Zoological Gardens, produced puppies which were in the first generation marked with white and other colors.

The following cases from De Varigny are also very striking. The dwarf trees from Japan, for the most part conifers, which may be a hundred years old and not be more than three feet high, are in part the result “of mechanical processes which prevent the spreading of the branches, and in part of a starving process which consists in cutting most roots and in keeping the plant in poor soil.”

As an example of the sudden appearance of a new variation the following case is interesting. A variety of begonia is recorded as having appeared quite suddenly at a number of places at the same time. In another case a narcissus which had met with adverse circumstances, and had then been supplied with a chemical manure in some quantity, began to bear double flowers.

Amongst animals the following cases of the appearance of sudden variations are pointed out by De Varigny. “In Paraguay, during the last century (1770), a bull was born without horns, although his ancestry was well provided with these appendages, and his progeny was also hornless, although at first he was mated with horned cows. If the horned and the hornless were met in fossil state, we would certainly wonder at not finding specimens provided with semi-degenerate horns, and representing the link between both, and if we were told that the hornless variety may have arisen suddenly, we should not believe it and we should be wrong. In South America also, between the sixteenth and eighteenth centuries the niata breed of oxen sprang into life, and this breed of bulldog oxen has thriven and become a new race. So in the San Paulo provinces of Brazil, a new breed of oxen suddenly appeared which was provided with truly enormous horns, the breed of franqueiros, as they are called. The mauchamp breed of sheep owes its origin to a single lamb that was born in 1828 from merino parents, but whose wool, instead of being curly like that of its parents, remained quite smooth. This sudden variation is often met with, and in France has been noticed in different herds.”

The ancon race of sheep originated in 1791 from a ram born in Massachusetts having short crooked legs and a long back. From this one ram by crossing, at first with common sheep, the ancon race has been produced. “When crossed with other breeds the offspring, with rare exception, instead of being intermediate in character, perfectly resemble either parent; even one of twins has resembled one parent and the second the other.”

Two especially remarkable cases remain to be described. These are the Porto Santo rabbit and the japanned peacock. Darwin has given a full account of both of these cases. “The rabbits which have become feral on the island of Porto Santo, near Madeira, deserve a fuller account. In 1418 or 1419 J. Gonzales Zarco happened to have a female rabbit on board which had produced young during the voyage, and he turned them all out on the island. These animals soon increased so rapidly that they became a nuisance, and actually caused the abandonment of the settlement. Thirty-seven years subsequently, Cada Mosto describes them as innumerable; nor is this surprising, as the island was not inhabited by any beast of prey, or by any terrestrial mammal. We do not know the character of the mother rabbit; but it was probably the common domestic kind. The Spanish peninsula, whence Zarco sailed, is known to have abounded with the common wild species at the most remote historical period; and as these rabbits were taken on board for food, it is improbable that they should have been of any peculiar breed. That the breed was well domesticated is shown by the doe having littered during the voyage. Mr. Wollaston, at my request, brought two of these feral rabbits in spirits of wine; and, subsequently, Mr. W. Haywood sent home three more specimens in brine and two alive. These seven specimens, though caught at different periods, closely resemble each other. They were full-grown, as shown, by the state of their bones. Although the conditions of life in Porto Santo are evidently highly favorable to rabbits, as proven by their extraordinarily rapid increase, yet they differ conspicuously in their small size from the wild English rabbit.... In color the Porto Santo rabbit differs considerably from the common rabbit; the upper surface is redder, and is rarely interspersed with any black or black-tipped hairs. The throat and certain parts of the under surface, instead of being pure white, are generally gray or leaden color. But the most remarkable difference is in the ears and tail. I have examined many fresh English rabbits, and the large collection of skins in the British Museum from various countries, and all have the upper surface of the tail and the tips of the ears clothed with blackish gray fur; and this is given in most works as one of the specific characters of the rabbit. Now in the seven Porto Santo rabbits the upper surface of the tail was reddish brown, and the tips of the ears had no trace of the black edging. But here we meet with a singular circumstance: in June, 1861, I examined two of these rabbits recently sent to the Zoological Gardens and their tails and ears were colored as just described; but when one of their dead bodies was sent to me in February, 1863, the ears were plainly edged, and the upper surface of the tail was covered with blackish gray fur, and the whole body was much less red; so that under the English climate this individual rabbit had recovered the proper color of its fur in rather less than four years.”

Another striking case of sudden variation is found in the peacock. It is all the more remarkable because this bird has hardly varied at all under domestication, and is almost exactly like the wild species living in India to-day. Darwin states: “There is one strange fact with respect to the peacock, namely, the occasional appearance in England of the ‘japanned’ or ‘black-shouldered’ kind. This form has lately been named, on the high authority of Mr. Slater, as a distinct species, viz. Pavo nigripennis, which he believes will hereafter be found wild in some country, but not in India, where it is certainly unknown. The males of these japanned birds differ conspicuously from the common peacock in the color of their secondary wing-feathers, scapulars, wing-coverts, and thighs, and are, I think, more beautiful; they are rather smaller than the common sort, and are always beaten by them in their battles, as I hear from the Hon. A. S. G. Canning. The females are much paler-colored than those of the common kind. Both sexes, as Mr. Canning informs me, are white when they leave the egg, and they differ from the young of the white variety only in having a peculiar pinkish tinge on their wings. These japanned birds, though appearing suddenly in flocks of the common kind, propagate their kind quite truly.”

In two cases, in which these birds had appeared quite suddenly in flocks of the ordinary kind, it is recorded that “though a smaller and weaker bird, it increased to the extinction of the previously existing breed.” Here we have certainly a remarkable case of a new species suddenly appearing and replacing the ordinary form, although the birds are smaller, and are beaten in their battles.