6.2 Tradition Comes Into the World¶
In the forests and following the grass over the Eocene plains there appeared for the first time a variety and abundance of mammals. Before we proceed to any description of these mammals, it may be well to note in general terms what a mammal is.
From the appearance of the vertebrated animals in the Lower Palaeozoic 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 mammae 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, but which lay eggs, the Ornithorhynchus (Platypus) 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.
Now we may put the essential facts about mammalian reproduction in another way. The mammal is a family animal. And the family habit involved the possibility of a new sort of continuity of experience in the world. Compare the completely closed-in life of an individual lizard with the life of even a quite lowly mammal of almost any kind. The former has no mental continuity with anything beyond itself; it is a little self-contained globe of experience that serves its purpose and ends; but the latter “picks up” from its mother, and “hands on” to its offspring. All the mammals, except for the two genera we have named, had already before the lower Eocene age arrived at this stage of pre-adult dependence and imitation.
They were all more or less imitative in youth and capable of a certain modicum of education; they all, as a part of their development, received a certain amount of care and example and even direction from their mother. This is as true of the hyena and rhinoceros as it is of the dog or man; the difference of educability is enormous, but the fact of protection and educability in the young stage is undeniable. So far as the vertebrated animals go, these new mammals, with their viviparous, young-protecting disposition, and these new birds, with their incubating, young-protecting disposition, introduce at the opening of the Cainozoic period a fresh thing into the expanding story of life, namely, social association, the addition to hard and inflexible instinct of tradition, and the nervous organisation necessary to receive tradition.
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 Paleozoic 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 Palaeozoic, 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 today only begins to appreciate the significance.
|||They secrete a nutritive fluid on which the young feeds from glands scattered over the skin. But the glands are not gathered together into mammae with nipples for suckling. The stuff oozes out, the mother lies on her back, and the young browse upon her moist skin.|