300 EVOLUTION IN BIOLOGY. [LECT. 



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. 



The conclusions enunciated by Cuvier and Von 

 Baer have been confirmed, in principle, by all subse- 

 quent 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 dis- 

 pose 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 stru cture. At the present moment, 

 it is impossible to draw up any definition, based on 

 broad anatomical or developmental characters, by 

 which any one of Cuvier's great groups shall be 

 separated from all the rest. On the contrary, the 

 lower members of each tend to converge towards the 

 lower members of all the others. The same may be 

 said of the vegetable world. The apparently clear 

 distinction between flowering and flowerless plants 

 has been broken down by the series of gradations 



