92 GENEEAL EYOLUTIOK 



VII. That the relations of the genera of a primary series are 

 those of the different steps in the development of the individuals 

 of the extreme genus ab ovo {von Baer^ Agassiz) (with sometimes 

 the addition of special adaptive features ?). 



VIII. That the presence, rudimental condition, or absence of 

 a given generic character can be accounted for on the hypothesis 

 of a greater rapidity of development in the individuals of the spe- 

 cies of the extreme type, such stimulus being more and more vig- 

 orous in the individuals of the types as we advance toward the 

 same, or by a reversed imj^ulse of development, where the extreme 

 is characterized by absence or "mutilation" of characters. 



IX. And that, as the character of the genus at the period of 

 reproduction of its species is that which is per^^etuated ; 



- X. So the character of the genus has been first inferior, then 

 protean, and then advanced, as the metamorphosis has been by a 

 retrograde movement in time, posterior to, at, or anterior to the 

 period of reproduction. 



XI. That it therefore results that there is one primary struct- 

 ural type involved in such a series of species, which is made to 

 present, at any given j)eriod in its geologic history, that appearance 

 of succession of genera ordained by Creative Power. 



d. On the Origin of Inexact Parallelism. 



The hypothesis can only be demonstrated in case of exact par- 

 allelism. If proved in these, it readily accounts for the cases of 

 inexact parallelism, which are of course in any single period vastly 

 in the majority. First take the case of simple inexact parallel- 

 ism. A series of individ uals of the genus Didocus undergo the 

 metamorphosis of the cranial structure earlier and earlier in life, 

 commencing by completing the ossification of the membrane of 

 the fronto-parietal region in full age, until at last it becomes com- 

 13leted as early as the period of reproduction. Heretofore the 

 adult offspring have appeared during a long period, invariably 

 characterized by the larval cranium ; but like now producing like, 

 this development springs into new power, and the offspring ossify 

 the cranial bones far earlier than their immediate predecessors ; 

 in a word, the genus Pelobates has been created ! At this state of 

 progress Didocus is an undeveloped Pelobates. 



Let us, however, suppose the '^acceleration " of development 

 of the cranial bones still to progress. The character appears now 

 soon after the ordinary metamorphosis has been passed, and now 



