Chap. XIII. 
EMBEYOLOGY. 
447 
sponding late age—the fore-limbs in the embryos of 
the several descendants of the parent-species will still 
resemble each other closely, for they will not have 
been modified. But in each of our new species, 
the embryonic fore-limbs will differ greatly from the 
fore-limbs in the mature animal; the limbs in the 
latter having undergone much modification at a 
rather late period of life, and having thus been con¬ 
verted into hands, or paddles,-or wings. Whatever 
influence long-continued exercise or use on the one 
hand, and disuse on the other, may have in modi¬ 
fying an organ, such influence will mainly affect the 
mature animal, which has come to its full powers of 
activity and has to gain its own living; and the effects 
thus produced will be inherited at a corresponding 
mature age. Whereas the young will remain unmodified, 
or be modified in a lesser degree, by the effects of use 
and disuse. 
In certain cases the successive steps of variation 
might supervene, from causes of which we are wholly 
ignorant, at a very early period of life, or each step 
might be inherited at an earlier period than that at 
which it first appeared. In either case (as with the 
short-faced tumbler) the young or embryo would closely 
resemble the mature parent-form. We have seen that 
this is the rule of development in certain whole groups 
of animals, as with cuttle-fish and spiders, and with a 
few members of the great class of insects, as with Aphis. 
With respect to the final cause of the young in these 
cases not undergoing any metamorphosis, or closely 
resembling their parents from their earliest age, we 
can see that this would result from the two following 
contingencies: firstly, from the young, during a course 
of modification carried on for many generations, having 
to provide for their own wants at a very *early stage 
