EVOLUTION IN BIOLOGY. 307 



assertion to such a series as may be formed out of the ab- 

 stractions constituted by the common characters of each 

 group.* 



Cuvier on anatomical, and Yon Baer on embryological 

 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 organi- 

 sation to be observed among them, no one of which, in 

 its highest and most complicated modification, leads to 

 any of the others. 



The conclusions enunciated by Cuvier and Yon Baer 

 have been confirmed, in principle, by all subsequent 

 research into the structure of animals and plants. But 

 the effect of the adoption of these conclusions has been 

 rather to substitute a new metaphor for that of Bonnet 

 than to abolish the conception expressed by it. Instead 

 of regarding living things as capable of arrangement in 

 one series like the steps of a ladder, the results of 

 modern investigation compel us to dispose them as if 

 they were the twigs and branches of a tree. The ends 

 of the twigs represent individuals, the smallest groups of 

 twigs species, larger groups genera, and so on, until we 

 arrive at the source of all these ramifications of the main 

 branch, which is represented by a common plan of struct- 

 ure. At the present moment, it is impossible to draw 

 up any definition, based on broad anatomical or develop- 



* " H s'agit done de prouver que la sdrie qui constitute l'e"chelle animale 

 reside essentiellement dans la distribution des masses principales qui la 

 composent et non dans celle des especes ni memo toujours dans celle des 

 genres." " Phil. Zoologique," chap. v. 



