162 THE ORIGIN OF SPECIES 



of grass would 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 ex- 

 tremely small area, especially if freely open to immigra- 

 tion, and where the contest between individual and 

 individual must be very severe, we always find great 

 diversity in its inhabitants. For instance, 1 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 (suj^posing 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 differ- 

 ences of habit and constitution, determine that the in- 

 habitants, 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 



