114 
NATUEAL SELECTIO:?^. 
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, thi^ee 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 
