216 EVOLUTION IN BIOLOGY vi 



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 regard- 

 ing 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 structure. 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 between 

 the two exhibited by the Lycopodiacece, HJiizo- 

 carpece, and Gymnospermece. The groups of Fungi, 

 Lichenes, and Algce have completely run into one 

 another, and, when the lowest forms of each are 



