The Evolution of Man, vol 2 by Ernst Haeckel (fun books to read for adults TXT) 📕

The phylogeny of the placenta has become more intelligible from the fact that we have found a number of transitional forms of it. Some of the Marsupials (Perameles) have the beginning of a placenta. In some of the Lemurs (Tarsius) a discoid placenta with decidua is developed.

While these important results of comparative embryology have been throwing further light on the close blood-relationship of man and the anthropoid apes in the last few years (Chapter 1.15), the great advance of paleontology has at the same time been affording us a deeper insight into the stem-history of the Placental group. In the seventh chapter of my Systematic Phylogeny of the Vertebrates I advanced the hypothesis that the Placentals form a single stem with many branches, which has been evolved from an older group of the Marsupials (Prodidelphia). The four great legions of the Placentals—Rodents, Ungulates, Carnassia, and Primates—are sharply separated to-day by important features of organisation. But if we consider their extinct ancestors of the Tertiary period, the differences gradually disappear, the deeper we go in the Cenozoic deposits; in the end we find that they vanish altogether. The primitive stem-forms of the Rodents (Esthonychida), the Ungulates (Chondylarthra), the Carnassia (Ictopsida), and the Primates (Lemuravida) are so closely related at the beginning of the Tertiary period that we might group them together as different families of one order, the Proplacentals (Mallotheria or Prochoriata).

Hence the great majority of the Placentals have no direct and close relationship to man, but only the legion of the Primates. This is now generally divided into three orders—the half-apes (Prosimiae), apes (Simiae), and man (Anthropi). The lemurs or half-apes are the stem-group, descending from the older Mallotheria of the Cretaceous period. From them the apes were evolved in the Tertiary period, and man was formed from these towards its close.

The Lemurs (Prosimiae) have few living representatives. But they are very interesting, and are the last survivors of a once extensive group. We find many fossil remains of them in the older Tertiary deposits of Europe and North America, in the Eocene and Miocene. We distinguish two sub-orders, the fossil Lemuravida and the modern Lemurogona. The earliest and most primitive forms of the Lemuravida are the Pachylemurs (Hypopsodina); they come next to the earliest Placentals (Prochoriata), and have the typical full dentition, with forty-four teeth (3.1.4.3. over 3.1.4.3.). The Necrolemurs (Adapida, Figure 2.274) have only forty teeth, and have lost an incisor in each jaw (2.1.4.3. over 2.1.4.3.). The dentition is still further reduced in the Lemurogona (Autolemures), which usually have only thirty-six teeth (2.1.3.3. over 2.1.3.3.). These living survivors are scattered far over the southern part of the Old World. Most of the species live in Madagascar, some in the Sunda Islands, others on the mainland of Asia and Africa. They are gloomy and melancholic animals; they live a quiet life, climbing trees, and eating fruit and insects. They are of different kinds. Some are closely related to the Marsupials (especially the opossum). Others (Macrotarsi) are nearer to the Insectivora, others again (Chiromys) to the Rodents. Some of the lemurs (Brachytarsi) approach closely to the true apes. The numerous fossil remains of half-apes and apes that have been recently found in the Tertiary deposits justify us in thinking that man’s ancestors were represented by several different species during this long period. Some of these were almost as big as men, such as the diluvial lemurogonon Megaladapis of Madagascar.

(FIGURE 2.276. The white-nosed ape (Cercopithecus petaurista).)

Next to the lemurs come the true apes (Simiae), the twenty-sixth stage in our ancestry. It has been beyond question for some time now that the apes approach nearest to man in every respect of all the animals. Just as the lowest apes come close to the lemurs, so the highest come next to man. When we carefully study the comparative anatomy of the apes and man, we can trace a gradual and uninterrupted advance in the organisation of the ape up to the purely human frame, and, after impartial examination of the “ape problem” that has been discussed of late years with such passionate interest, we come infallibly to the important conclusion, first formulated by Huxley in 1863: “Whatever systems of organs we take, the comparison of their modifications in the series of apes leads to the same result: that the anatomic differences that separate man from the gorilla and chimpanzee are not as great as those that separate the gorilla from the lower apes.” Translated into phylogenetic language, this “pithecometra-law,” formulated in such masterly fashion by Huxley, is quite equivalent to the popular saying: “Man is descended from the apes.”

(FIGURE 2.277. The drill-baboon (Cynocephalus leucophaeus) (From Brehm.))

In the very first exposition of his profound natural classification (1735) Linne placed the anthropoid mammals at the head of the animal kingdom, with three genera: man, the ape, and the sloth. He afterwards called them the “Primates”—the “lords” of the animal world; he then also separated the lemur from the true ape, and rejected the sloth. Later zoologists divided the order of Primates. First the Gottingen anatomist, Blumenbach, founded a special order for man, which he called Bimana (“two-handed”); in a second order he united the apes and lemurs under the name of Quadrumana (“four-handed”); and a third order was formed of the distantly-related Chiroptera (bats, etc.). The separation of the Bimana and Quadrumana was retained by Cuvier and most of the subsequent zoologists. It seems to be extremely important, but, as a matter of fact, it is totally wrong. This was first shown in 1863 by Huxley, in his famous Man’s Place in Nature. On the strength of careful comparative anatomical research he proved that the apes are just as truly “two-handed” as man; or, if we prefer to reverse it, that man is as truly four-handed as the ape. He showed convincingly that the ideas of hand and foot had been wrongly defined, and had been improperly based on physiological instead of morphological grounds. The circumstance that we oppose the thumb to the other four fingers in our hand, and so can grasp things, seemed to be a special distinction of the hand in contrast to the foot, in which the corresponding great toe cannot be opposed in this way to the others. But the apes can grasp with the hind-foot as well as the fore, and so were regarded as quadrumanous. However, the inability to grasp that we find in the foot of civilised man is a consequence of the habit of clothing it with tight coverings for thousands of years. Many of the bare-footed lower races of men, especially among the negroes, use the foot very freely in the same way as the hand. As a result of early habit and continued practice, they can grasp with the foot (in climbing trees, for instance) just as well as with the hand. Even new-born infants of our own race can grasp very strongly with the great toe, and hold a spoon with it as firmly as with the hand. Hence the physiological distinction between hand and foot can neither be pressed very far, nor has it a scientific basis. We must look to morphological characters.

As a matter of fact, it is possible to draw such a sharp morphological distinction—a distinction based on anatomic structure—between the fore and hind extremity. In the formation both of the bony skeleton and of the muscles that are connected with the hand and foot before and behind there are material and constant differences; and these are found both in man and the ape. For instance, the number and arrangement of the smaller bones of the hand and foot are quite different. There are similar constant differences in the muscles. The hind extremity always has three muscles (a short flexor muscle, a short extensor muscle, and a long calf-muscle) that are not found in the fore extremity. The arrangement of the muscles also is different before and behind. These characteristic differences between the fore and hind extremities are found in man as well as the ape. There can be no doubt, therefore, that the ape’s foot deserves that name just as much as the human foot does, and that all true apes are just as “bimanous” as man. The common distinction of the apes as “quadrumanous” is altogether wrong morphologically.

But it may be asked whether, quite apart from this, we can find any other features that distinguish man more sharply from the ape than the various species of apes are distinguished from each other. Huxley gave so complete and demonstrative a reply to this question that the opposition still raised on many sides is absolutely without foundation. On the ground of careful comparative anatomical research, Huxley proved that in all morphological respects the differences between the highest and lowest apes are greater than the corresponding differences between the highest apes and man. He thus restored Linne’s order of the Primates (excluding the bats), and divided it into three sub-orders, the first composed of the half-apes (Lemuridae), the second of the true apes (Simiadae), the third of men (Anthropidae).

But, as we wish to proceed quite consistently and impartially on the laws of systematic logic, we may, on the strength of Huxley’s own law, go a good deal farther in this division. We are justified in going at least one important step farther, and assigning man his natural place within one of the sections of the order of apes. All the features that characterise this group of apes are found in man, and not found in the other apes. We do not seem to be justified, therefore, in founding for man a special order distinct from the apes.

The order of the true apes (Simiae or Pitheca)—excluding the lemurs—has long been divided into two principal groups, which also differ in their geographical distribution. One group (Hesperopitheca, or western apes) live in America. The other group, to which man belongs, are the Eopitheca or eastern apes; they are found in Asia and Africa, and were formerly in Europe. All the eastern apes agree with man in the features that are chiefly used in zoological classification to distinguish between the two simian groups, especially in the dentition. The objection might be raised that the teeth are too subordinate an organ physiologically for us to lay stress on them in so important a question. But there is a good reason for it; it is with perfect justice that zoologists have for more than a century paid particular attention to the teeth in the systematic division and arrangement of the orders of mammals. The number, form, and arrangement of the teeth are much more faithfully inherited in the various orders than most other characters.

Hence the form of dentition in man is very important. In the fully developed condition we have thirty-two teeth; of these eight are incisors, four canine, and twenty molars. The eight incisors, in the middle of the jaws, have certain characteristic differences above and below. In the upper jaw the inner incisors are larger than the outer; in the lower jaw the inner are the smaller. Next to these, at each side of both jaws, is a canine (or “eye tooth”), which is larger than the incisors. Sometimes it is very prominent in man, as it is in most apes and many of the other mammals, and forms a sort of tusk. Next to this there are five molars above and below on each side, the first two of which (the “premolars”) are small, have only one root, and are included in the change of teeth; the three back ones are much larger, have two roots, and only come with the second teeth. The apes of the Old World, or all