Associated life meets us very early in the story of animal existence, and we may assume that it ranks as a genuine "survival quality." It enables some animals to survive the attacks of others that are individually stronger, and it may even be, as has been suggested, that associated life is the normal form, and that solitary animals represent a variation from the normal, or perhaps a case of degeneration. But one result of associated life is that it paves the way for the emergence of mind as an active force in social evolution. In his suggestive and important work on _Mutual Aid_, Kropotkin has well shown how in the animal world the purely biologic form of the struggle for existence is checked and transformed by the factors of mutual aid, association and protection. His illustrations cover a very wide field; they include a great variety of animal forms, and he may fairly claim to have established the proposition that "an instinct has been slowly developed among animals and men in the course of an extremely long evolution ... which has taught animals and men alike the force they can borrow from mutual aid and support, and the joys they can find in social life." But there is, on the whole, a very sharp limit set to the development of mind in the animal world. One cause of this is the absence of a true "social medium," to use the admirable phrase of that versatile thinker, George Henry Lewes. In the case of man, speech and writing enable him to give to his advances and discoveries a cumulative force such as can never exist in their absence. On that subject more will be said later. At present we may note another very important consequence of the development of mind in evolution. In pre-human, or sub-human society, perfection in the struggle for existence takes the form of the creation or the perfecting of an organic tool. Teeth or claws become stronger or larger, a limb is modified, sight becomes keener, or there is a new effect in coloration. The changes here, it will be observed, are all of an organic kind, they are a part of the animal and are inseparable from it, and they are only transmissible by biologic heredity. And the rate of development is, of necessity, slow.