124 ORIGIN OF SPECIES 



otherwise natural selection can effect nothing. So it will be 

 with plants. It has been experimentally proved, that if a 

 plot of ground be sown with one species of grass, and a similar 

 plot be sown with several distinct genera of grasses, a greater 

 number of plants and a greater weight of dry herbage can 

 be raised in the latter than in the former case. The same 

 has been found to hold good when one variety and several 

 mixed varieties of wheat have been sown on equal spaces of 

 ground. Hence, if any one species of grass were to go on 

 varying, and the varieties were continually selected which 

 differed from each other in the same manner, though in a 

 very slight degree, as do the distinct species and genera of 

 grasses, a greater number of individual plants of this species, 

 including its modified descendants, would succeed in 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 distinct varieties of 

 any one species of grass would have the best chance of suc- 

 ceeding 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 se- 

 vere, 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 dififered 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 



