CJiAP. IV.] EVOLUTION THE KEY TO DISTRIBUTION. 



57 



The Amount of Variation in North American Birds. — An 

 American naturalist, Mr. J. A. Allen, has made elaborate 

 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 distri- 

 bution 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 considerable, 

 often reaching to one-sixth or one-seventh of the average 

 dimensions, and sometimes more. Thus Turdus fuscescens 

 (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 Dendroeca 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 Melosjnza melodia (the 

 American song-sparrow) being sometimes reduced to narrow 

 lines, while in other specimens they are so enlarged as to cover 

 the greater part of the breast and sides of the body, sometimes 

 uniting on the middle of the breast into a nearly continuous 

 patch. In one of the small spotted wood-thrushes, Turdus 

 fuscescens, the colours are sometimes very pale, and the mark- 

 ings on the breast reduced to indistinct narrow lines, while in 

 other specimens the general colour is much darker, and the 

 breast markings dark, broad, and triangular. All the variations 

 here mentioned occur between adult males, so that there is no 



