THE ORIGIN OF SPECIES AND GENERA 293 



than now sometimes occurs in the same litter. It is often 

 thought, however, that wild animals do not vary suffi- 

 ciently to enable any such change as this to be brought 

 about in the same limited time ; and though naturalists 

 are well aware that there is little, if any, difference in 

 this respect between wild and domesticated species, it is 

 only recently that they have been able to adduce positive 

 proof that this is the case. 



We owe this proof to an American naturalist, Mr. J. A. 

 Allen,^ who was the first to make a series of observations 

 and measurements of the mammals, and more especially 

 of the birds, of the United States ; and he found a wonder- 

 ful and altogether unsuspected amount of variation 

 between individuals of the same species even when in- 

 habiting the same locality. They differ in the general 

 tint, and in the distribution of the colours and markings ; 

 in general size, and in proportions ; in the length of the 

 head, feet, wings, and tail ; in the length of particular 

 feathers, thus altering the shape of the wing or tail ; in 

 the length of the tarsi and of the separate toes ; and in 

 the length, width, thickness, and curvature of the bill. 

 These variations are by no means small in amount or 

 requiring very accurate measurements for their detection, 

 since they often reach one-seventh, one-sixth, or some- 

 times even one-fourth, of the entire average dimensions. 

 Thus, in twelve species of small birds, the variation in 

 twenty-five or thirty specimens, all taken in the same 

 locality, and of the same sex and age was, in the length 

 of the folded wing, from 14*5 to 21 per cent., and in the 

 length of the tail from 14 to 23'4 per cent. If we take 

 individual cases, we find equally striking facts. Wilson's 

 thrush (Turdus fuscesccns) was found to vary in length of 

 wing from 3*58 to 415 inches, and in the tail from 3"55 

 to 4 inches. In the Blue-bird (Sicdia sialu) the middle 

 toe varied from 0*77 to 0'91 inch, and the hind toe from 

 0-58 to 0-72 inch ; while the bill varied from 0-45 to 



^ "On the Mammals and Winter Birds of East Florida; with an 

 Examination of certain assumed Specific Characters in Birds, and a 

 Sketch of the Bird Fauna; of Eastern North America." By J. A. Allen. 

 (Bulletin of the Museum of Comparative Zoology at Harvard College, 

 Cambridge, Mass., vol. ii. No. 3.) 



