VON ZITTEL'S "HANDBUCH" 117 



Notwithstanding these difficulties, it is possible, 

 however, to cite, among fossil animals, a few cases 

 of the preservation of embryonic stages. The 

 Palaeozoic Belenuridae resemble the young larva of 

 the existing Limulus. The pentacrinoi'd larva of the 

 Antedon is more akin to certain fossil Crinoids than 

 is the adult animal. Several fossil Sea-urchins retain 

 the permanently linear ambulacra and pentangular 

 peristome which are the transitory and adolescent 

 characteristics of their existing descendants. Even 

 in groups entirely extinct, some series of ontogenic 

 characteristics may be sometimes successfully re- 

 traced ; the fine researches of Hyatt, Wurtem- 

 berger, and Branco have shown that the Ammonites 

 and Ceratites pass through a Goniatite stage, 

 and that the internal whorls of some genera of 

 Ammonite reproduce, in a transitory state, the 

 form, the ornamentation, and the lines of suture 

 possessed, in the adult state, by certain other 

 genera of a former geological epoch. 



The series of forms of which the consecutive links 

 accord with the adolescent stages of more recent 

 types do not offer us only an image of the progress of 

 development of a given group. The reconstruction 

 of such genealogical trees form the most important 

 desideratum in palaeontology. But we are yet 

 very far from the goal. Nothing shows more 

 how arbitrary these genealogies are than the un- 

 satisfactory state of our knowledge as to the evolu- 

 tion of the great Ammonite group. 



Finally, von Zittel terminates with an admirable 

 appeal to prudence : 



" The theory of descent," he says, " has intro- 



