350 THE CLASSICAL TRADITION 



activity. This moment is naturally dependent upon the 

 external conditions. Among the highest Vertebrates, the 

 mammals, the traces of phylogeny shown in ontogeny are to 

 a great extent obliterated through the adaptation of ontogeny 

 to the external conditions, and through the modifications 

 which the germs of more highly organised animals necessarily 

 exhibit from the very beginning as compared with germs 

 which do not reach such a high level of development " 

 (p. 754, iSQ7). 



Study of individual variation in the time of appearance 

 of the organs in embryos of the same species was prosecuted 

 with interesting results by Bonnet, 1 Mehnert,- and Fischel. :! 

 Fischel found that variability was greatest among the 

 younger embryos, and became progressively less in later 

 stages. Like von Baer (s/t/>nr, p. 114) he inferred that 

 regulatory processes were at work during development 

 which brought divergent organs back to the normal and 

 enabled them to play their part as correlated members of 

 a functional whole. 



Important theoretical views were developed by Mehnert ' 

 in a series of publications appearing from 1891 to 1898. 

 Like Keibel, Mehnert emphasised the importance of function 

 in determining the late or early appearance of organs, but he 

 conceived the influence of function to be exerted not only in 

 ontogeny, but also throughout the whole course of phylogeny, 

 by reason of the transmission to descendants of the effects of 

 functioning in the individual life. 



In his paper of 1897 Mehnert details the results of an 

 extensive examination of the development of the extremities 

 throughout the Amniote series. lie finds that in all cases 

 a pentadactylate rudiment is formed, even in those forms in 



1 " Hcitra'tfe zur Embryologie der Wiederkaucr," Arch. Anat. Fnfw., 

 1889. 



"Die Individ. Variation d. Wirbeltiercmbryo," Morf>h. Arbeit., v., 

 1895. 



"U. Yariabilitat u. YVaclistum d. embryonalen Korpers," Morph. 

 J<ihrb., xxiv., i. 



"(i.-tstrulation u. Keimblatterbildun- dcr Emyx /nf t in\i fuuri, ;/,' 

 Morfih. Arbeit., i., i8;i. " Kain.^cnrsr," .l/,v///. Arbeit., vii., pp. 1-156, 

 i . and .ilsn M-p.u.itely. Hioniefhiinik, crsclilosscn ,mx <l,-i 1'rin //, 

 . fena, r 



