482 POPULAR SCIENCE MONTHLY 



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 arti- 

 ficial. 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 meas- 

 urements 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 independ- 

 ently 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 



