246 FOOT-NOTES TO EVOLUTION. 



stock of unspecialized Prohcanitida endured as long as 

 the race. The number of phylogerontic forms increases 

 in the Mesozoic, showing a constantly increasing tend- 

 ency to become abnormal, until before the end of the 

 Cretaceous the entire race of ammonoids becomes phy- 

 logerontic, and dies out from sheer lack of plasticity to 

 modify itself further with changing conditions. 



Such a general view or family tree of the ammonoids 

 may be seen in any of the text-books of paleontology, 

 especially those of Steinmann,* and of K. von Zittel,f 

 where we get the best attempts to represent our present 

 knowledge and ideas of the genetic relationships of am- 

 monites. These genealogies are, however, purely ten- 

 tative, based not on ontogeny but on comparison of 

 series of adults. This would undoubtedly be the safest 

 way if we had a perfect series of genera and species, but 

 such a thing is unknown, and can never be obtained, on 

 account of the incompleteness of the geologic record, 

 and the mixing of faunas by migration in the past. 



The researches of Hyatt, Branco, and Karpinsky 

 have given us a surer way ; from their work we have 

 learned that the Ammonoidea preserve in each individual 

 a complete record of their larval and early adolescent 

 history, the embryonic protoconch and the young cham- 

 bers being enveloped and protected by later stages of 

 the shell. Also the record is a perfect one, for no 

 resorption of stages of the shell has ever been observed 

 in the chambered cephalopods. And so by breaking off 

 the outer chambers the naturalist can in effect cause the 

 shell to repeat its life history in inverse order, for each 

 stage of growth represents some extinct ancestral genus. 

 These genera appeared in the exact order of their 

 minute imitations in the larval history of their descend- 



* Elemente der Palaeontologie, 1890. 

 f Grundziige der Palaeontologie, 1895. 



