y its parents. Hence, I conclude, that it is quite possible,
that each of the many successive modifications, by which each species
has acquired its present structure, may have supervened at a not very
early period of life; and some direct evidence from our domestic animals
supports this view. But in other cases it is quite possible that each
successive modification, or most of them, may have appeared at an
extremely early period.

I have stated in the first chapter, that there is some evidence to
render it probable, that at whatever age any variation first appears
in the parent, it tends to reappear at a corresponding age in the
offspring. Certain variations can only appear at corresponding ages, for
instance, peculiarities in the caterpillar, cocoon, or imago states of
the silk-moth; or, again, in the horns of almost full-grown cattle. But
further than this, variations which, for all that we can see, might have
appeared earlier or later 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 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 amou