VI EVOLUTION IN BIOLOGY 215 



But., as knowledge advanced, this conception 

 ceased to be tenable in the crude form in which 

 it was first put forward. Taking into account 

 existing; animals and plants alone, it became 

 obvious that they fell into groups which were 

 more or less sharply separated from one another ; 

 and, moreover, that even the species of a genus 

 can hardly ever be arranged in linear series. 

 Their natural resemblances and differences are 

 only to be expressed by disposing them as if they 

 were branches springing from a common hypo- 

 thetical centre. 



Lamarck, while affirming the verbal proposition 

 that animals form a single series, was forced by his 

 vast acquaintance with the details of zoology to 

 limit the assertion to such a series as may be 

 formed out of the abstractions constituted by the 

 common characters of each group. 1 



Cuvier on anatomical, and Von Baer on embryo- 

 logical grounds, made the further step of proving 

 that, even in this limited sense, animals cannot be 

 arranged in a single series, but that there are 

 several distinct plans of organisation to be observed 

 among them, no one of which, in its highest and 

 most complicated modification, leads to any of the 

 others. 



1 "II s'agit done de prouver que la serie qui constitue 

 P&helle aniniale reside cssentiellcmcnt dans la distribution des 

 masses principales qui la composent et non dans celle des especes 

 ni meme toujours dans celle des genres." Philosophic Zoologique, 

 chap. v. 



43 



