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 orthogenetic, and that the inherited impulse towards determinate modifications 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 ontogenetic method has given rise to their false attitude towards the doctrine 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 enormously 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 postembryonic 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 ontogeny of all individuals. Moreover, in closely related members of one genetic group, the duration of these stages and substages is approximately uniform. Change in form, however, may vary greatly, and have
