444 
EMBKYOLOGY. 
Chap. XIII. 
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 reajipear 
at a corresponding age in the offspring. Certain varia¬ 
tions can only appear at corresponding ages, for in¬ 
stance, peculiarities in the caterpillar, cocoon, or imago 
states of the silk-moth; or, again, in the horns of almost 
full-grown cattle. But farther 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 bull¬ 
dog, 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 differ¬ 
ence. 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 
