From the appearance of the vertebrated animals in the Lower Palæozoic Age, when the fish first swarmed out into the sea, there has been a steady progressive development of vertebrated creatures. A fish is a vertebrated animal that breathes by gills and can live only in water. An amphibian may be described as a fish that has added to its gill-breathing the power of breathing air with its swimming-bladder in adult life, and that has also developed limbs with five toes to them in place of the fins of a fish. A tadpole is for a time a fish; it becomes a land creature as it develops. A reptile is a further stage in this detachment from water; it is an amphibian that is no longer amphibious; it passes through its tadpole stage--its fish stage, that is--in an egg. From the beginning it must breathe in air; it can never breathe under water as a tadpole can do. Now, a modern mammal is really a sort of reptile that has developed a peculiarly effective protective covering, hair; and that also retains its eggs in the body until they hatch so that it brings forth living young (viviparous), and even after birth it cares for them and feeds them by its mammæ for a longer or shorter period. Some reptiles, some vipers for example, are viviparous, but none stand by their young as the real mammals do. Both the birds and the mammals, which escaped whatever destructive forces made an end to the Mesozoic reptiles, and which survived to dominate the Cainozoic world, have these two things in common: first, a far more effective protection against changes of temperature than any other variation of the reptile type ever produced; and, secondly, a peculiar care for their eggs, the bird by incubation and the mammal by retention, and a disposition to look after the young for a certain period after hatching or birth. There is by comparison the greatest carelessness about offspring in the reptile. Hair was evidently the earliest distinction of the mammals from the rest of the reptiles. It is doubtful if the particular Theriodont reptiles who were developing hair in the early Mesozoic were viviparous. Two mammals survive to this day which not only do not suckle their young,[17] but which lay eggs, the _Ornithorhynchus_ and the _Echidna_, and in the Eocene there were a number of allied forms. They are the survivors of what was probably a much larger number and variety of small egg-laying hairy creatures, hairy reptiles, hoppers, climbers, and runners, which included the Mesozoic ancestors of all existing mammals up to and including man.