68 



ISLAND LIFE 



1>ART t 



ably find that in no one generation was there a greater 

 difference than now occurs in the same breed, or sometimes 

 even the same Utter. It is often thought, however, that 

 wild species do not vary sufficiently to bring about any 

 such change as this in the same time ; and though 

 naturalists are well aware that this is a mistake, it is only 

 recently that they have been able to adduce positive proof 

 of their opinion. 



The Amount of Variation in JSorth American Birds. — 

 An American naturalist, Mr. J. A. Allen, has made elabor- 

 ate observations and measurements of the birds of the 

 United States, and he finds a wonderful and altogether 

 unsuspected amount of variation between individuals of 

 the same species. They differ in the general tint, and in 

 the markings and distribution of the colours ; in size and 

 proportions ; in the length of the wings, tail, bill, and feet ; 

 in the length of particular feathers, 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 very consider- 

 able, often reaching to one-sixth or one-seventh of the 

 average dimensions, and sometimes more. Thus Turdus 

 fmcescens (Wilson's thrush) varied in length of wing from 

 3'58 to 4*16 inches, and in the tail from 3*55 to 4*00 inches ; 

 and in twelve specimens, all taken in the same locality, 

 the wing varied in length from 14*5 to 21 per cent., and 

 the tail from 14 to 22-5 per cent. In Sialia sialis (the 

 blue bird) the middle toe varied from '77 to '91 inch, and 

 the hind toe from '58 to '72 inch, or more than 21*5 per 

 cent, on the mean, while the bill varied from '45 to '56 

 inch in length, and from '30 to '38 inch in width, or about 

 20 per cent, in both cases. In Bendrxca coronata (the 

 yellow-crowned warbler) the quills vary in proportionate 

 length, so that the 1st, the 2nd, the 3rd, or the 4th, is 

 sometimes longest; and a similar variation of the wing 

 involving a change of proportion between two or more of 

 the feathers is recorded in eleven species of birds. Colour 

 and marking vary to an equal extent ; the dark streaks on 

 the under surface of Melospiza melodia (the American 

 song-sparrow) being sometimes reduced to narrow lines. 



