At the present time the mammals are the highest and on the whole the most successful of the vertebrate groups. They include the largest and strongest, the swiftest-footed and the most intelligent of the animal kind. They show refinements of the senses of sight, hearing, and smell such as are met with nowhere else. They range from the Equator to the coldest regions of the earth in which any food is to be found; they people alike the forest and the plain, and have their representatives both in the air and in the sea.
Fig. 105.—The Australian Duck-mole or Duck-billed Platypus.
The most lowly of the mammals are the Monotremes, which include the well-known Australian duck-mole or duck-billed platypus, and two species of spiny ant-eaters, one of which is found in Australia, the other in New Guinea. The two types are shown in Figs. 105 to 107. The best-known and most striking fact concerning these is that, like the birds and reptiles, and unlike all other mammals, they lay eggs. Beyond this feature they show many affinities with the reptiles, in their skeleton for example, and particularly in their reproductive organs. Another interesting fact is that the blood temperature, of the ant-eater at least, is low, and varies considerably. It has been found to range between 80 and 93 degrees Fahrenheit, whereas all other mammals, so far as is known, have a blood temperature between 97 and 105, and moreover one that is remarkably constant for each species during normal health. The brain development and the intelligence are also much lower than in other mammals, though distinctly superior to those met with among the reptiles. The method of rearing the young in the case of the ant-eater (that of the duck-mole not being yet fully known) is that the single egg is placed, as soon as it is laid, in the pouch under the belly of the female. Here it hatches in a very short time, and here the young animal remains for the first two or three months of its life, being nourished by milk produced by the mammary glands, which open into the pouch. It is certainly owing to the absence of the competition of the higher mammals in the regions where they are found, that these two creatures, so interesting from the standpoint of Evolution, have been preserved to us.
Fig. 106.—New Guinea Spiny Ant-eater.
Fig. 107.—Australian Spiny Ant-eater.
The next group, the Marsupials, is the lowest in which we get the true mammalian characteristic of the bearing of living young. For while occasional members of other groups, of the reptiles especially, produce live young, the actual state of affairs is fundamentally different. In these latter the egg is merely retained in the genital duct until the young creature emerges. It is merely hatched inside the body of the mother instead of outside. But in the Marsupials the developing young receive nourishment from the mother, during prenatal life, other than what is contained in the yolk. This nourishment is obtained in the form of a secretion from the wall of the uterus, there being as yet (with a single partial exception) no real connection between mother and young.
The peculiar method of nourishing and protecting the young Marsupial after birth is of course well known. The young are born in a very immature and helpless condition, and are placed by the mother in her pouch. The mouth becomes permanently attached to the nipple of the dam, and the young creature remains thus for a considerable time, the milk being pumped into it by the mammary gland rather than sucked in by the efforts of the creature itself.