446 
EMBRYOLOGY. 
Chap. XIII, 
later in life, if the full-grown animal possesses them. 
And the cases just given, more especially that of 
pigeons, seem to show that the characteristic differences 
which give value to each breed, and which have been 
accumulated by man’s selection, have not generally first 
appeared at an early period of life, and have been in¬ 
herited by the offspring at a corresponding not early 
period. But the case of the short-faced tumbler, which 
when twelve hours old had acquired its proper propor¬ 
tions, proves that this is not the universal rule; for here> 
the characteristic differences must either have appeared 
at an earlier period than usual, or, if not so, the differ¬ 
ences must have been inherited, not at the corresponding, 
but at an earlier age. 
Now let us apply these facts and the above two 
principles—which latter, though not proved true, can 
be shown to be in some degree probable—to species 
in a state of nature. Let us take a genus of birds, 
descended on my theory from some one parent-species, 
and of which the several new species have become 
modified through natural selection in accordance with 
their diverse habits. Then, from the many slight suc¬ 
cessive steps of variation having supervened at a rather 
late age, and having been inherited at a corresponding 
age, the young of the new species of our supposed genus 
will manifestly tend to resemble each other much more 
closely than do the adults, just as we have seen in the 
case of pigeons. We may extend this view to whole 
families or even classes. The fore-limbs, for instance, 
which served as legs in the parent-species, may have 
become, by a long course of modification, adapted in 
one descendant to act as hands, in another as paddles, 
in another as wings; and on the above two principles— 
namely of each successive modification supervening at 
a rather late age, and being inherited at a corre- 
