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.