NATURAL SELECTION 103 



applies to one animal will apply throughout all time 

 to all animals — that is, if they vary — for otherwise 

 natural selection can do 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 thus be raised. The same 

 has been found to hold good when first one variety 

 and then 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 those 

 varieties were continually selected which differed from 

 each other in at all the same manner as distinct species 

 and genera of grasses differ from each other, a greater 

 number of individual plants of this species of grass, 

 including 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 

 annually sowing almost countless seeds ; and thus, as 

 it may be said, is striving its utmost to increase its 

 numbers. Consequently, 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 in- 

 creasing 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, three feet by four in size, which had 

 been exposed for many years to exactly the same con- 

 ditions, 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. 



