in life, tend to appear at a corresponding age in the offspring and
parent. I am far from meaning that this is invariably the case; and I could
give a good many cases of variations (taking the word in the largest sense)
which have supervened at an earlier age in the child than in the parent.

These two principles, if their truth be admitted, will, I believe, explain
all the above specified leading facts in embryology. But first let us look
at a few analogous cases in domestic varieties. Some authors who have
written on Dogs, maintain that the greyhound and bulldog, though appearing
so different, are really varieties most closely allied, and have probably
descended from the same wild stock; hence I was curious to see how far
their puppies differed from each other: I was told by breeders that they
differed just as much as their parents, and this, judging by the eye,
seemed almost to be the case; but on actually measuring the old dogs and
their six-days old puppies, I found that the puppies had not nearly
acquired their full amount of proportional difference. So, again, I was
told that the foals of cart and race-horses differed as much as the
full-grown animals; and this surprised me greatly, as I think it probable
that the difference between these two breeds has been wholly {445} caused
by selection under domestication; but having had careful measurements made
of the dam and of a three-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 carefully
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