Chap. IV. Divergence of Character. 89 



piece of ground, could live on it (supposing its nature not to be in 

 any way peculiar), and may be said to be striving to the utmost to 

 live there ; but, it is seen, that where they come into the closest 

 competition, the advantages of diversification of structure, with the 

 ■accompanying differences of habit and constitution, determine 

 .that the inhabitants, which thus jostle each other most closely, 

 shall, as a general rule, belong to what we call different genera 

 and orders. 



The same principle is seen in the naturalisation of plants through 

 man's agency in foreign lands. It might have been expected that 

 the plants which would succeed 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 naturalised genera, no less than 100 genera are not there indi- 

 genous, and thus a large proportional addition is made to the genera 

 now living in the United States. 



By considering the nature of the plants or animals which have in 

 any country struggled successfully with the indigenes, and have 

 there become naturalised, we may gain some crude idea in what 

 manner some of the natives would have to be modified, in order to 

 gain an advantage over their compatriots ; and we may at least 

 infer that diversification of structure, amounting to new generic 

 differences, would be profitable to them. 



The advantage of diversification of structure 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 Milne Edwards. No physiologist 

 •doubts that a stomach adapted to digest vegetable matter alone, or 

 flesh alone, draws most nutriment from these substances. So in the 

 general economy of any land, the more widely and perfectly the 

 animals and plants are diversified for different habits of life, so will 



