1 ] 2 DIVERGENCE OF CHARACTER. Chap. IV, 



lUT as distiiKl species and gwiora of grasses differ from each 

 other, a greater miinber of individual phmts of this species of 

 grass, inchiding its modified descendants, ■would succeed in 

 living on the same piece of ground. And we well know that 

 each species and each variety of grass is aimually sowing al- 

 most countless seeds : and thus, as it may be said, is striving 

 its utmost to increase its numbers. Consequently, in the course 

 of many thousand generations, the most distinct varieties of 

 any one species of gTass would always have the best chance of 

 succeeding and of increasing in numbers, and thus of supplant- 

 ing 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 

 iinder many natural circumstances. In an extremely small 

 area, especially if freely open to immigration, and where the 

 contest between indivitkial and individual must be severe, we 

 always find groat 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 shoAvs how much 

 these plants differ from each other. So it is with tlie plants 

 and insects on small and imiform 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 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 

 tliey come into the closest competition with each other, the 

 advantages of diversification of structure, with the accom])any- 

 ing difVerenccs 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 

 expected that the plants which would succeed in becoming 

 naturalized in any land would generally have been closely 

 aUied to tlie indigenes ; for these are commonly looked at as 

 specially created and adapted for their own country. It might 



