296 THE POPULAR SCIENCE MONTHLY 



of opinion is due to a difference of method. "When the student of shells 

 directs his attention chiefly to adult characters, this definitely directed 

 variation, independent of environment, is not recognized by him. But 

 no one can study the details of shell ontogeny, especially in the earlier 

 stages, without quickly realizing that ontogenetic development is ortho- 

 genetic, and that the inherited impulse towards determinate modifica- 

 tions is the most powerful controlling factor of the animal's life history. 



So far as invertebrates are concerned, the study of post-embryonic 

 development was first seriously undertaken by the immortal Hyatt, in 

 his work on the ammonites. To be sure, others before him — notably 

 d'Orbigny — noticed that a distinct series of changes was recognizable in 

 the shell of ammonites, but no one before Hyatt actually employed this 

 method. He himself once told me that when, in the early sixties, he 

 first realized the importance of this method of study when actually 

 applied to shelled organisms, and its value as a guide in phylogeny, 

 it seemed so marvelously simple that he felt sure that the method and 

 its application must be fully understood by all working naturalists. 

 " But," he added, " I soon found that I practically stood alone, and I 

 have spent my life since in the endeavor to convert them to my point 

 of view." 



This misunderstanding, on the part of many zoologists, of the onto- 

 genetic method has given rise to their false attitude towards the doc- 

 trine of the recapitulation of ancestral characters. This subject will 

 be adequately treated by some of my successors, but I can not forbear 

 to anticipate them to the extent of pointing out this fact: When the 

 embryologist seeks for proof or disproof of this concept in the enor- 

 mously condensed record of the stages between the ovum and birth, he 

 is bound to be grievously disappointed; for this record, necessarily 

 modified by eliminations, can only furnish general resemblances of the 

 embryo to earlier types, and can not be said to actually recapitulate the 

 life history of the entire race. When, however, the student of post- 

 embryonic ontogeny compares the youthful stages of an individual with 

 the adult of immediately preceding species of the same genetic series, 

 the fact of recapitulation becomes at once apparent. 



The post-embryonic life history of an individual falls readily into 

 stages, of which four major ones have been recognized and named, 

 chiefly by Hyatt. These are: (1) the infant or nepionic stage; (2) 

 the adolescent or neanic stage; (3) the adult or ephebic stage, and 

 (4) the senile or gerontic stage, followed by death. These onto-stages, 

 as they may be called, are further divided into substages, designated by 

 the prefixes ana, meta and para, and they may be observed in the ontog- 

 eny of all individuals. Moreover, in closely related members of one 

 genetic group, the duration of these stages and substages is approxi- 

 mately uniform. Change in form, however, may vary greatly, and have 



