CHAr. 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 4T6 
inches, and in the tail from 3*55 to 400 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 Melospiza 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 
