No. 579] PROGRESSIVE EVOLUTION 



163 



We also know that even the most highly specialized 

 air-breathing vertebrates, which never live in water and 

 never require gills or gill-slits at all, nevertheless pos- 

 sess very distinct gill-slits during a certain period of 

 their development. This is one of the most familiar il- 

 lustrations of the law of recapitulation, and my only 

 excuse for bringing it forward now is tliat T wish, before 

 going further, to consider a difficulty — perhaps more ap- 

 parent than real — that arises in connection with such 

 cases. 



It might be argued that if gill-slits arose in response 

 to the stimuli of aquatic life, and if these stimuli are no 

 longer operative in the case of air-breathing vertebrates, 

 then gill-slits ought not to be developed at any stage of 

 their existence. This argument is, I think, fully met by 

 the following considerations. 



At any given moment of ontogenetic development the 

 condition of any organ is merely the last term of a series 

 of morphogenetic stages, while its environment at the 

 same moment — which, of course, includes its relation to 

 all the other organs of the body — is likewise merely the 

 last term of a series of environmental stages. We have 

 thus two parallel series of events to take into considera- 

 tion in endeavoring to account for the condition of any 

 part of an organism— or of the organism as a whole — at 

 any period of its existence: 



Ej E 2 E 3 ... E» environmental stages, 

 M, M 2 M 3 ... Mn morphogenetic stages. 



Ontogeny is absolutely conditioned by the proper cor- 

 relation of the stages of these two series at every point, 

 and hence it is that any sudden change of environment 

 is usually attended by disastrous consequences. Thus, 

 after the fish-like ancestors of air-breathing vertebrates 

 had left the water and become amphibians, they doubt- 

 less still had to go back to the water to lay their eggs, 

 in order that the eggs might have the proper conditions 

 for their development. 



