NATURAL SELECTION 89 



plants, and these belonged to eighteen genera and to eight orders, 

 which shows how much these plants differed from 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 

 more 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 naturalization of plants 

 through man's agency in foreign lands. It might have been ex- 

 pected that the plants which would succeed in becoming natural- 

 ized 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 county. It might also, perhaps, have 

 been expected that naturalized 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 naturalization, 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 

 naturalized plants are enumerated, and these belong to 162 genera. 

 We thus see that these naturalized plants are of a highly diversified 

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

 for out of the 162 naturalized genera, no less than 100 genera are 

 not there indigenous, 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 naturalized, 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 



