assigned to the formerly popular argument of the "missing links." The wonder is not that so many links are missing, but that any of these primitive forms have come down to us. For we see here another proof of the fearful extermination of lower forms during the progress of life on the globe. It seems as if the intermediate forms were less common among these most recent animals than among the older types. This may not be true, for it is not easy to compare the gap between two mammals with that between two worms or insects, and mistakes are very easily made. But it seems as if extermination had done its work more ruthlessly among these highest forms than among their humbler and lower ancestors. I would not lay much weight on such an opinion; but, if true, it has a meaning and is worthy of study.

In higher, true, placental mammals the period of pregnancy is much longer, and the young are born in a far higher stage of development, or rather, growth. The stage of growth at which the young are born differs markedly in different groups. A new-born kitten is a much feebler, less developed being than a newborn calf. An embryonic appendage, the allantois, used in reptiles and birds for respiration, has here been turned to another purpose. It lays itself against the walls of the uterus, uterine projections interlock with those which it puts forth, and the blood of the mother circulates through a host of capillaries separated from those of the blood system of the embryo only by the thinnest membrane. This is the placenta, developed, in part from the allantois of the embryo, in part from the uterus of the mother. It is not a new organ, but an old one turned to better and fuller use.

In these closely associated systems of blood-vessels, nutriment and oxygen diffuse from the blood of the mother into that of the embryo, and thus rapid growth is assured. The importance and far-reaching effect of this new modification in the old reproductive system cannot be over-estimated. The internal intra-uterine development of the young, and the mammalian habit of suckling them, far more than any other factors, have made man what he is. Some explanation must be sought for such a fact.

We have already seen that any animal devotes to reproduction the balance between income and expenditure of nutriment. Now, the digestive system is here well developed, and the income is large. But we have already noticed that, as animals grow larger, the ratio between the digestive surface and the mass to be supported grows continually smaller. On account of size alone the mammal has but a small balance. But the amount of expenditure is proportional to the mass and activity of the muscular and nervous systems. And the mammal is, and from the beginning had to be, an exceedingly active, energetic, and nervous animal. The income has increased, but the expenses have far outrun the increase. The mammal can devote but little to reproduction.

Moreover, it requires a large amount of material to form a mammalian egg, such as that of the monotreme. It requires indefinitely more nutriment to build a mammal than a worm, for the former is not only larger and more perfect at birth; it is also vastly more complicated. The embryonic journey has, so to speak, lengthened out immensely. One monotreme egg represents more economy and saving than a thousand

eggs of a worm. Moreover, where the individuals are longer lived and the generations follow one another at longer intervals, the number of favorable variations and the possibility of conformity to environment through these is greatly lessened. In such a group it is of the utmost importance that every egg should develop; the destruction of a single one is a real and important loss to the species. It is not enough to produce such an egg; it must be most scrupulously guarded. Even the egg of the platypus is deposited in a nest in a hole in the bank, and the female Echidna carries the egg in a marsupial pouch until it develops.

Notice further that among certain species of fish, amphibia, and reptiles, the females carry the eggs in the body until the embryos or young are fairly developed. Viviparous forms are unknown by birds, probably because this mode of development is incompatible with flight, their dominant characteristic. Putting these facts together, what more probable than that certain primitive egg-laying mammals should have carried the eggs as long as possible in the uterus. The embryo under these conditions would be better nourished by a secretion of the uterine glands than by a very large amount of yolk. The yolk would diminish and the egg decrease in size, and thus the marsupial mode of development would have resulted. And, given the marsupial mode of development and an embryo possessing an allantois, it is almost a physiological necessity that in some forms at least a placenta should develop. That the placenta has resulted from some such process of evolution is proven by its different stages of development in different orders of mammals. And even the feeblest attachment of the allantois of the embryo to

the wall of the uterus would be of the greatest advantage to the species.

This is not the whole explanation; other factors still undiscovered were undoubtedly concerned. But even this shows us that the internal development of the young and the habit of suckling them was a logical result of mammalian structure and position. The grand results of this change we shall trace farther on.

The changes from the lower true mammals to the apes are of great interest, but we can notice only one or two of the more important. The prosimii, or "half apes," including the lemurs, are nearly all arboreal forms. Perhaps they were driven to this life by their more powerful competitors. The arboreal life developed the fingers and toes, and most of these end, not with a claw, but with a nail. The little group has much diversity of structure, and at present finds its home mainly in Madagascar; though in earlier times apparently occurring all over the globe. The brain is more highly developed than in the average mammal, but far inferior to that of the apes. They have a fairly opposable thumb.

The highest mammals are the primates. Their characteristics are the following: Fingers and toes all armed with nails, the eyes comparatively near together and fully enclosed in a bony case. The cerebrum with well-developed furrows covers the other portions of the brain. There is but one pair of milk-glands, and these on the breast. The differences between hand and foot become most strongly marked by the "anthropoid" apes. These have become accustomed to an upright gait in their climbing; hence the feet are used for supporting the body and the hands for grasping. Both

thumb and great toe are opposable; but the foot is a true foot, and the hand a true hand, in anatomical structure. The face, hands, and feet have mainly lost the covering of hair. They have no tail, or rather its rudiments are concealed beneath the skin. These include the gibbon, the orang, the gorilla, and the chimpanzee.

We can sum up the few attainments of mammals in a line. The lower forms attained the placental mode of embryonic development; the higher attained upright gait, hands and feet, and a great increase of brain. Anatomically considered these were but trifles, but the addition of these trifles revolutionized life on the globe. The principal anatomical differences between man and the anthropoid ape are the following: Man is a strictly erect animal. The foot of the ape is less fitted for walking on the ground, where he usually 'goes on all fours." The skull is almost balanced on the condyles by which it articulates with the neck, and has but slight tendency to tip forward. The facial portion, nose and jaws, is less developed and retracted beneath the larger cranium or brain-case. This has greatly changed the appearance of the head. Protruding jaws and chin, even when combined with large cranium and brain, always give man the appearance of brutality and low intelligence.

The pelvis is broad and comparatively shallow. The legs, especially the thighs, are long. The foot is long and strong, and rests its lower surface, not merely the outer margin as in apes, on the ground. The elastic arch of the instep must be excepted in the above description, and adds lightness and swiftness to his otherwise slow gait. The great toe is short and gener