Chap. XIII. 
EMBBYOLOGY. 
445 
caused by selection under domestication; but haying 
had careful measurements made of the dam and of a 
tln^ee-days old colt of a race and heavy cart-horse, I find 
that the colts have by no means acquired their full 
amount of proportional difference. 
As the evidence appears to me conclusive, that the 
several domestic breeds of Pigeon have descended from 
one wild species, I compared young pigeons of various 
breeds, within twelve hours after being hatched; I care¬ 
fully measured the proportions (but will not here give 
details) of the beak, width of mouth, length of nostril 
and of eyelid, size of feet and length of leg, in the 
wild stock, in pouters, fantails, runts, barbs, dragons, 
carriers, and tumblers. Now some of these birds, when 
mature, differ so extraordinarily in length and form 
of beak, that they would, I cannot doubt, be ranked in 
distinct genera, had they been natural productions. 
But when the nestling birds of these several breeds 
were placed in a row, though most of them could be 
distinguished from each other, yet their proportional 
differences in the above specified several points were 
incomparably less than in the full-grown birds. Some 
characteristic points of difference—for instance, that of 
the width of mouth—could hardly be detected in the 
young. But there was one remarkable exception to this 
rule, for the young of the short-faced tumbler differed 
from the young of the wild rock-pigeon and of the other 
breeds, in all its proportions, almost exactly as much as 
in the adult state. 
The two principles above given seem to me to explain 
these facts in regard to the later embryonic stages of 
our domestic varieties. Fanciers select their horses, 
dogs, and pigeons, for breeding, when they are nearly 
grown up : they are indifferent whether the desired 
qualities and structures have been acquired earlier or 
