240 SMITH. [Vol. XVI. 



off one at a time, until all were gone before the end of that 

 age. Only the simple radicles or stocks persisted, but from 

 time to time certain genera branched off from the main line, 

 became highly specialized, and often gave rise to so-called 

 abnormal forms, such as Hamites, Baadites, Crioceras^ Scapkites, 

 phylogerontic or degenerate genera (retrogressive), which did 

 not perpetuate their race, but soon died out without descendants. 



Of course there were many other phylogerontic genera that 

 were not abnormal in form ; thus Clymenia branched off in the 

 Upper Devonian into a variety of species, and disappeared as 

 suddenly ; Medlicottia reached its culmination in the Permian, 

 barely managed to live on into the Trias, and disappeared with- 

 out posterity ; while the main stock of unspecialized Prolecani- 

 tidae endured as long as the race. 



In the beginning the number of phylogerontic forms was 

 small, for most of the goniatites left descendants among the 

 ammonites ; but their number increased during the Mesozoic, 

 showing a constantly growing tendency to become abnormal, 

 until before the end of the Cretaceous the entire stock of am- 

 monites had become phylogerontic, and died out finally from 

 sheer lack of plasticity to modify itself further with changing 

 conditions. 



The life history of the ammonites is a finished chapter in 

 biology, and we have in museums and monographs a nearly 

 complete record of their development. It only remains to 

 study genetic series of ammonites. One way (that usually 

 adopted) is to compare a series of adults from successive 

 geologic periods, and by tracing resemblances to construct 

 theoretical family trees. Results of this work may be seen 

 in text-books of paleontology, and its unreliability may be real- 

 ized if one ever tries to use these tentative genealogies. This 

 would undoubtedly be the safer way if we had a complete geo- 

 logic record and if the faunas of the various geographic 

 provinces had been preserved. But since this is not the case, 

 we have to meet conditions as they are, not as they might be. 



Palingenesis. — The other way is to study the ontogeny of 

 representative species under each genus, and by comparing 

 each stage of growth with antecedent forms to find out the 



