212 DEVELOPMENT A/VD HEREDITY. 



change the movements in the middle of the passage 

 will be resisted by the strength of the associations, 

 but by a continued change of the stimulus this 

 resistance will be overcome, the movements will 

 be changed, and new associations will be established. 

 The modified series differs from the original series 

 by only a few links in one part of the chain. The 

 method and nature of the associations in this illustra- 

 tion and in the hereditary impulse, I believe to be 

 essentially the same. 



In the same manner may be explained all the 

 temporary embryonic organs and conditions of 

 growth which exist in the embryo, but which can- 

 not be regarded as ever having existed in any adult 

 ancestral form. Thus the gill-clefts of the mammals 

 can be regarded as inherited from some adult fish- 

 like ancestors, but not the placenta ; and the first 

 stage of the cranial flexure we cannot imagine to 

 have ever existed in any adult animals. The pla- 

 centa is undoubtedly the result of its immediate 

 surroundings, while the cranial flexure seems to be 

 due to its peculiar method of rapid increase of bulk, 

 and to be the manner of closing the neural canal. 



Another class of phenomena suggests itself here, 

 as being somewhat akin to the phenomena of em- 

 bryonic growth ; namely, the development of larval 

 forms and metamorphosis. An examination of the 



