ORGANIC EVOLUTION — PHYSICAL 51 



individual all the vast numbers of variations whicli 

 occurred among the millions of ancestors from the 

 unicellular organism downwards can be recapitulated ; 

 and (2) as regards the highest anunals, that it is obvious 

 that the embryo in its various stages cannot present 

 exact copies of its remote ancestors, for the simple 

 reason, that in its various stages it is incapable of living 

 apart from the parent. For instance, no animal that 

 even approximately resembled the human embryo of 

 two months could be capable of living outside the body 

 of its mother. But these objections are met when we 

 take into consideration what is undoubtedly true, viz. 

 that variations occur, and that natural selection (re- 

 versed selection in this case), and particularly cessation 

 of natural selection, act, not only at the end of the 

 ontogeny, but during the entire period of development ; 

 natural selection during that period seizing upon as 

 favourable and accumulating all such variations as tend 

 to shorten and simplify the process of development, and 

 cessation of natural selection tending to bring about 

 the disappearance of all characters which during the 

 phylogeny had been useful, but which to the embryo, 

 living under different conditions, protected and sustained 

 as it is within the body of its parent in the highest 

 animals, or advantageously placed as regards the en- 

 vironment by the parent in the case of lower animals, 

 are no longer usefuL 



One other point remains to be cleared up. It has 

 been observed that the embryo of a high animal, c, g. 

 mam, resembles the embryos of lower animals, never 

 the lower animals themselves ; for instance, the embryo 

 of a man in one stage of its ontogeny resembles the 

 embryo of a frog, never the adult frog. The reason for 

 this is obvious ; man has not descended from the frog, 

 but the frog and man have descended from a common 

 ancestry, and therefore both animals as they recapitulate 



