Page:Popular Science Monthly Volume 68.djvu/486

482 have been termed fluctuations. These occur in every individual, in every organ and practically in every direction. They are traceable, on the one hand, to differences in the germ cell, and in the process of mitosis or cleavage to which it is subject, and, on the other, to the fact of amphimixis, or double parentage, universal in the higher animals and plants. These fluctuations are hereditary, but their existence is easily obscured by the fact that the slight variations in the one parent rarely coincide with those of the other. In the progress of a species, individual fluctuations tend to neutralize one another.

It is, however, certain that fluctuations can be rapidly intensified and rendered stable by the process of selection, either natural or artificial. While it is probably true that few species originate by the process of selection alone, it is almost certain that it is possible for selection alone to produce groups equivalent to those we call species. There is apparently no limit to what man can do by the persistent preservation of favorable or desirable fluctuations. The same results must occur in nature when the same process takes place if it ever does take place. A slight advantage on the side of any special fluctuation will change the average variation in that direction and in time make the character permanent.

One of the first noteworthy studies of fluctuations as distinguished from climatic, environmental and geographic variations was that made by Dr. J. A. Allen in his paper on the 'Mammals and Winter Birds of East Florida,' published in 1871. In this paper Dr. Allen gives measurements of many specimens of various species of common birds, with a view to ascertaining the normal rate of variation. In this regard, the study of birds is much more instructive than that of most groups, because a bird of any species rapidly reaches a natural stature. It is affected in its growth by environment or food conditions much less than the members of most other groups. The well-fed bird reaches its normal stature, the ill-fed or injured bird dies, and the bird changes little with growth, its adult condition being once attained. Among adult male birds, characters due to accidents of condition are reduced to the minimum.

Dr. Allen finds that each part is subject to an ordinary variation of from 15 to 20 per cent, in its measurements, this in specimens of the same age and sex; at the same time each part varies independently of the others, even each feather of the wing and tail. Each toe may vary for itself, and the bill and the claws are subject to the same deviations from the normal. Color variations are equally well marked, and the variations within the species as represented in a single locality are often as great as those which actually distinguish species. In birds with streaks or spots, these markings vary in size, form and number, each individual having its own traits, which persist through the seasonal changes of plumage. It is part of the art of the faunal