402 ON THE ORIGIN OF SPECIES 



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 diifer 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 converted 

 into hands, or paddles, or wings. Whatever influ- 

 ence long-continued exercise or use on the one hand, 

 and disuse on the other, may have in modifying an 

 organ, such influence will mainly affect the mature 

 animal, which has come to its full powers of activity 

 and has to gaiu 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 

 of development, and secondly, from their following 

 exactly the same habits of life with their parents ; for 

 in this case, it would be indispensable for the existence 

 of the species, that the child should be modified at aj 

 very early age in the same manner with its parents, in 

 accordance with their similar habits. Some further 



