JAMES PERRIN SMITH 19 



But in all speculations on the phylogeny of Cretaceous genera we 

 must not forget that there still exists a great gap in our knowledge of 

 the connections between Triassic and later groups, and that some of the 

 stocks may possibly have lived on in unknown regions, to reappear in 

 later ages so greatly modified that their ancestral history comes out 

 only in their reversion to the parent type, when senescence has awakened 

 the latent tendencies of their far distant youth. A case that may illus- 

 trate this is the parallelism of Paratissotia of the Cretaceous, PL X, figs. 

 8-10, with Otoceras of the Lower Triassic, PL X, figs. 6 and 7. Otoceras 

 belongs to the family Hungaritidae, the most ancient line of the Cera- 

 titoidea, and Paratissotia belongs to the Amaltheidae, which are thought 

 by some to have come from the Ceratitidae. In this case the parallelism 

 may be due to atavism. 



In the cases discussed above, generic persistence from the Permian 

 until the Upper Cretaceous is out of the question, and even the families 

 referred to have not outlived the Triassic in most cases. But the 

 Cretaceous forms must have had Paleozoic and early i\Iesozoic ancestors 

 which were in the transition between the goniatitic, ceratitic and 

 ammonitic stages of development. And being all somewhat retarded, 

 and in most cases affected by arrest of development, it is highly probable 

 that they would revert to some of the characters of those remote pro- 

 genitors. 



Genetic Series. 



Ever since the acceptance of the theory of evolution, genetic series 

 have been sought by geologists with more or less success. Waagen's 

 studies in the Formenreihe of Oppelia, and Hyatt's "Genesis of the 

 Arietidae" have become classic. But some more conservative paleon- 

 tologists have always cherished secret doubts of the demonstration, while 

 admitting the truth of the principle. It is extremely doubtful if we can 

 establish any genetic lines of species, or that we can ever tell from 

 which particular species a certain genus originated. Did it, indeed, 

 come from only one? What the paleontologist sees is rather a group 

 of species tending in somewhat the same direction; and those species 

 most alike he classes, for convenience, under one genus. Further, the 

 conservative paleontologist can not always point to the individual genus 

 from which another genus sprang; and if he does he is probably mistaken. 

 Every virile progressive stock is characterized by its wealth in variation, 

 its genera and species as we grade them, any one of which, or all of 

 which, might have been ancestors of later forms. 



