Chap. IV. DIVERGENCE OF CHARACTER. 115 



plants through man's agency in foreign lands. It 

 might have been expected that the plants which have 

 succeeded in becoming naturalised in any land would 

 generally have been closely allied to the indigenes ; for 

 these are commonly looked at as specially created and 

 adapted for their own country. It might, also, perhaps 

 have been expected that naturalised plants would have 

 belonged to a few groups more especially adapted to 

 certain stations in their new homes. But the case is 

 very different ; and Alph. De Candolle has well remarked 

 in his great and admirable work, that floras gain by 

 naturalisation, proportionally with the number of the 

 native genera and species, far more in new genera than 

 in new species. To give a single instance : in the last 

 edition of Dr. Asa Gray's 'Manual of the Flora of the 

 Northern United States,' 260 naturalised plants are 

 enumerated, and these belong to 162 genera. We thus 

 see that these naturalised plants are of a highly diversified 

 nature. They differ, moreover, to a large extent from 

 the indigenes, for out of the 162 genera, no less than 100 

 genera are not there indigenous, and thus a large pro- 

 portional addition is made to the genera of these States. 



By considering the nature of the plants or animals 

 which have struggled successfully with the indigenes of 

 any country, and have there become naturalised, we 

 can gain some crude idea in what manner some of the 

 natives would have had to be modified, in order to have 

 gained an advantage over the other natives ; and we 

 may, I think, at least safely infer that diversification of 

 structure, amoimting to new generic differences, would 

 have been profitable to them. 



The advantage of diversification in the inhabitants of 

 the same region is, in fact, the same as that of the 

 physiological division of labour in the organs of the 

 same individual body — a subject so well elucidated by 



