No. 579] PROGRESSIVE EVOLUTION 165 



for which the young were retained growing gradually 

 longer and longer. It is obvious that this change of 

 habit involves a corresponding change in the environ- 

 mental conditions under which the young develop, and 

 in cases in which the young are not born until they have 

 reached practically the adult condition this change di- 

 rectly affects practically the whole ontogeny. We may 

 say that the series 



Ei E 2 E 3 ... En has become 



E/E 2 'E 3 ' ... E/, 

 and as the change of environment must produce its ef- 

 fect upon the developing organism the series 



Mj M 2 M 3 ... M n will have become 



M/M 2 'M 3 ' ... M n \ 

 We must remember that throughout the whole course 

 of phylogenetic evolution this series is constantly length- 

 ening, so that what was the adult condition at one time 

 becomes an embryonic stage in future generations, and 

 the series thus represents not only the ontogeny, but 

 also, though in a more or less imperfect manner, the 

 phylogeny of the organism. 



The character of each stage in ontogeny must depend 

 upon (1) the morphological and physiological constitu- 

 tion of the preceding stage, and (2) the nature of the 

 environment in which development is taking place. We 

 can not, however, distinguish sharply between those two 

 sets of factors, for, in a certain sense, the environment 

 gradually becomes incorporated in the organism itself 

 as development proceeds, each part contributing to the 

 environment of all the remainder, and the influence of 

 this internal portion of the environment ever becoming 

 more and more important. 



The whole process of evolution depends upon changes 

 of environment taking place so gradually that the neces- 

 sary self-adjustment of the organism at every stage is 

 possible. In the case of our amphibia the eggs could 



