Page:A Short History of the World.djvu/58

 38 of learning by imitation, of communication, by warning cries and other concerted action, of mutual control and instruction. A teachable type of life had come into the world.

The earliest mammals of the Cainozoic period are but little superior in brain size to the more active carnivorous dinosaurs, but as we read on through the record towards modern times we find, in every tribe and race of the mammalian animals, a steady universal increase in brain capacity. For instance we find at a comparatively early stage, that rhinoceros-like beasts appear. There is a creature, the Titano- therium, which lived in the earliest division of this period. It was probably very like a modern rhinoceros in its habits and needs. But its brain capacity was not one tenth that of its living successor.

The earlier mammals probably parted from their offspring as soon as suckling was over, but, once the capacity for mutual under- standing has arisen, the advantages of continuing the association are very great ; and we presently find a number of mammalian species displaying the beginnings of a true social life and keeping together in herds, packs and flocks, watching each other, imitating each other, taking warning from each other's acts and cries. This is something that the world had not seen before among vertebrated animals. Reptiles and fish may no doubt be found in swarms and shoals ; they have been hatched in quantities and similar conditions have kept them together, but in the case of the social and gregarious mammals the association arises not simply from a community of external forces, it is sustained by an inner impulse. They are not merely like one another and so found in the same places at the same times ; they like one another and so they keep together.

This difference between the reptile world and the world of our human minds is one our sympathies seem unable to pass. We can- not conceive in ourselves the swift uncomplicated urgency of a rep- tile's instinctive motives, its appetites, fears and hates. We cannot understand them in their simplicity because all our motives are com- plicated ; ours are balances and resultants and not simple urgencies. But the mammals and birds have self-restraint and consideration for other individuals, a social appeal, a self-control that is, at its lower level, after our own fashion. We can in consequence establish relations with almost all sorts of them. When they suffer they utter cries and make movements that rouse our feelings. We can make understanding pets of them with a mutual recognition. They can be tamed to self-restraint towards us, domesticated and taught.