It must be obvious to every one, that in a field of a certain size, beside the corn-plants which have been sown, a great number of weeds can exist, and, moreover, in places which could not have been occupied by corn-plants. The more dry and sterile places of the ground, in which no corn-plant would thrive, may still furnish sustenance to weeds of different kinds; and such species and individuals of weeds will more readily be able to exist in such conditions, in proportion as they are suited to adapt themselves to the different parts of the ground. It is the same with animals. It is evident that a much greater number of animal individuals can live together in one and the same limited district, if they are of various and different natures, than if they are all alike. There are trees (for example, the oak) on which a couple of hundred of different species of insects live together. Some feed on the fruits of the tree, others on the leaves, others again on the bark, the root, etc. It would be quite impossible for an equal number of individuals to live on this tree if all were of one species; if, for example, all fed on the bark, or only upon the leaves. Exactly the same is the case in human society. In one and the same small town, only a certain number of workmen can exist, even when they follow different occupations. The division of labour, which is of the greatest use to the whole community, as well as to the individual workman, is a direct consequence of the struggle for life, of natural selection; for this struggle can be sustained more easily the more the activities, and hence, also, the forms of the different individuals deviate from one another. The different function naturally produces its reaction in changing the form, and the physiological division of labour necessarily determines the morphological differentiation, that is, the “divergence of character.”(37)