ducing themselves and handing on their qualities and impulses in whom the impulse of self-maintenance is not able to diminish the impulse to reproduce and protect the progeny.
4.—The Social Instinct.
Beside these instincts which are peculiar to the higher animals, the struggle for life develops in particular kinds of animals still others, which are special and conditioned by the peculiarity of their method of life; for example, the migratory instinct, which we will not further study. Here we are interested in another kind of instinct, which is of very great importance for our subject: the social instinct.
The co-operation of similar organisms in larger crowds is a phenomenon which we can discover quite in their earliest stages in the microbes. It is explained alone by the simple fact of reproduction. If the organisms have no self-movement, the progeny will, consequently, gather round the producer, if they are not by any chance borne away by the movements of the external world matter: currents, winds, and phenomena of that sort. The apple falls, as is well known, not far from the stem, and when it is not eaten, and falls on fruitful soil, there grow from its pips young trees, which keep the old tree company. But even in animals with power of self-movement it is very natural that the young should remain with the old if no external circumstances supply a ground for them to remove themselves. The living together of individuals of the same species, the most primitive form of social life, is also the most primitive form of life itself. The division of organisms, having common origin is a later act.
The separation can be brought about by the most diverse causes. The most obvious, and certainly the most effective, is the lack of sustenance. Each locality can only yield a certain quantity of food. If a certain species of animals multiplies over the limits of their food supply, the superfluous ones must either
