All the innovations that come into the history of life begin very humbly. The supply of blood-vessels in the swimming-bladder of the mudfish in the lower Palæozoic torrent-river, that enabled it to pull through a season of drought, would have seemed at that time to that bodiless visitant to our planet we have already imagined, a very unimportant side fact in that ancient world of great sharks and plated fishes, sea-scorpions, and coral reefs and seaweed; but it opened the narrow way by which the land vertebrates arose to predominance. The mudfish would have seemed then a poor refugee from the too crowded and aggressive life of the sea. But once lungs were launched into the world, every line of descent that had lungs went on improving them. So, too, in the upper Palæozoic, the fact that some of the Amphibia were losing their “amphibiousness” by a retardation of hatching of their eggs, would have appeared a mere response to the distressful dangers that threatened the young tadpole. Yet that prepared the conquest of the dry land for the triumphant multitude of the Mesozoic reptiles. It opened a new direction towards a free and vigorous land-life along which all the reptilian animals moved. And this viviparous, young-tending training that the ancestral mammalia underwent during that age of inferiority and hardship for them, set going in the world a new continuity of perception, of which even man to-day only begins to appreciate the significance. § 3 A number of types of mammal already appear in the Eocene. Some are differentiating in one direction, and some in another, some are perfecting themselves as herbivorous quadrupeds, some leap and climb among the trees, some turn back to the water to swim, but all types are unconsciously exploiting and developing the brain which is the instrument of this new power of acquisition and educability. In the Eocene rocks are found small early predecessors of the horse (Eohippus), tiny camels, pigs, early tapirs, early hedgehogs, monkeys and lemurs, opossums and carnivores. Now, all these were more or less ancestral to living forms, and all have brains relatively much smaller than their living representatives. There is, for instance, an early rhinoceros, _Titanotherium_, with a brain not one tenth the size of that of the existing rhinoceros. The latter is by no means a perfect type of the attentive and submissive student, but even so it is ten times more observant and teachable than its predecessor. This sort of thing is true of all the orders and families that survive until to-day. All the Cainozoic mammals were doing this one thing in common under the urgency of a common necessity; they were all growing brain. It was a parallel advance. In the same order or family to-day, the brain is usually from six to ten times what it was in the Eocene ancestor.