CHAP. IV.] DIVERGENCE OF CHARACTER. 83 



each other. So it is with the plants and insects on small and 

 uniform islets : also in small ponds of fresh water. Farmers find 

 that they can raise most food by a rotation of plants belonging to 

 the most different orders : nature follows what may be called a 

 simultaneous rotation. Most of the animals and plants which 

 live close round any small 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 indigenous, and thus a large pro- 

 portional 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, 



