92 CONTROLLED NATURAL SELECTION 



was present in the ancestors. Is it not reason- 

 able to suppose that only those characters are 

 retained which are useful, or that of a given 

 character only so much as is useful : or else 

 why are so many completely lost ? 



The Tadpole (Rana temporaries) has first 

 external gills, then internal gills, and finally 

 lungs, all of which are put to use : so too it 

 is possible that the gill clefts or their rudi- 

 ments found in the human embryo then serve 

 some useful purpose. Natural Selection must 

 deal with ova, embryos, juveniles, as it does 

 with adults, and produce embryonic or juvenile 

 characters. Ova and embryos, like adults, 

 have environments to which they must adapt 

 themselves. 



We are accustomed to 'study chiefly adult 

 characters and to forget that adult is but a 

 stage, merging on the one side into youth, and 

 on the other, old age ; each of which have 

 important characters. Some of the characters 

 of old age may be looked upon as means by 

 which the species rids itself of an individual 

 which has shed all its germ cells. 



Selection must occur with reference to a 

 character, at the time of, or after, the appear- 

 ance of the character, and not before. Thus 

 the characters that go to form an embryo or 



