NATURAL SELECTION 161 



succeed in increasing (the country not undergoing any 

 change in conditions) only by its varying descendants 

 seizing on places at present occupied by other animals: 

 some of them, for instance, being enabled to feed on new 

 kinds of prey, either dead or alive; some inhabiting new 

 stations, climbing trees, frequenting water, and some per- 

 haps becoming less carnivorous. The more diversified in 

 habits and structure the descendants of our carnivorous 

 animals become, the more places they will be enabled to 

 occupy. What applies to one animal will apply through- 

 out all time to ail animals — that is, if they vary — for 

 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 cau 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 vary- 

 ing, 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 an- 

 nually sowing almost countless seeds; and is thus striv- 

 ing, 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 





