104 ON THE ORIGIN OF SPECIES 



So it is with the plants and insects on small and 

 uniform islets ; and so 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 

 it not to be in any way peculiar in its nature), and may 

 be said to be striving to the utmost to live there ; but, 

 it is seen, that where they come into the closest com- 

 petition with each other, the advantages of diversifica- 

 tion 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 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 indi- 

 genous, and thus a large proportional addition is made 

 to the genera of these States. 



