What applies to one animal will apply throughout all time to all 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 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.