114 NATUKAL SELECTION. Chap. IV. 



sequently, I cannot doubt that in the course of many 

 thousands of generations, the most distinct varieties of 

 any one species of grass would always have the best 

 chance of succeeding and of increasing in numbers, and 

 thus of supplanting the 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 

 structure, is seen under many natural circumstances. 

 In an extremely small area, especially if freely open to 

 immigration, and where the contest between individual 

 and individual must be 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 ; 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 competition with each other, the advantages of 

 diversification of structure, with the accompanying dif- 

 ferences 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 



