138 DIVBRaBNCB OF CHARACTER. [Chap. IV. 



less distinct varieties; and varieties, when rendered very 

 distinct from each other, take the rank of species. 



The truth of the principle that the greatest amount 

 of life can be supported by great diversification of struc- 

 ture, is seen under many natural circumstances. In an 

 extremely small area, especially if freely open to immi- 

 gration, and where the contest between individual and 

 individual must be very severe, we always find great 

 diversity in its inhabitants. For instance, I found that 

 a piece of turf, three feet by four in size, which had been 

 exposed for many years to exactly the same conditions, 

 supported twenty species of 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 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 



