98 DIVERGENCE OF CHARACTER. 



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

 each species and each variety of grass is annually sowing 

 almost countless seeds ; and is thus striving, as it may be 

 said, to the utmost to increase in number. Consequently, 

 in the course of many thousand generations, the most dis* 

 tinct varieties of any one 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 extremely 

 small area, especially if freely open to immigration, and 

 where the contest between individual and individual must 

 be very severe, we always find great diversity in its inhab- 

 itants. 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 more 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 (suppos- 

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

 said to be striving to the utmost to live there ; but it i^ 

 seen, that where they come into the closest competition, the 

 advantages of diversification of structure, with the accom- 

 panying 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 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 allied to the indigenes ; for these are commonly 

 looked at as specially created and adapted for their own 

 county. It might also, perhaps, have been expected that nat- 

 uralized plants woujd have belonged to a few groups more 



