First, we have to consider the relative place which comparative anatomy concedes to man in the 'natural system' of animals, for the true value of our 'natural classification' is based upon its meaning as a pedigree. All the minor and major groups of the system—the classes, legions, orders, families, genera, and species—are only different branches of the same pedigree. For man himself, his place in the pedigree has been fixed since Lamarck,[5] in 1801, defined the group of vertebrates. The most perfect[6] of these are the Mammalia; and at the head of this class stands the order of Primates, in which LinnÆus, in 1735, united four 'genera'—Homo, Simia, Lemur, and Vespertilio. If we exclude the last-named, the Chiroptera of modern zoology, there remain three natural groups of Primates—the Lemures, the SimiÆ, and the Anthropi or HominidÆ. This is the classification of the majority of zoologists; but if we compare man with the two chief groups of monkeys—the Eastern monkeys (or CatarrhinÆ) and the Western or American monkeys (PlatyrrhinÆ)—there can be no doubt that the former group is much more closely related to man than is the latter. In the natural order of the CatarrhinÆ we find united a long series of lower and higher forms. The lowest, the Cynopitheci, appear still closely related to the PlatyrrhinÆ and to the Lemures; while, on the other hand, the tailless apes (AnthropomorphÆ) approach man through their higher organization. Hence one of our best authorities on the Primates, Robert Hartmann,[7] proposed to subdivide the whole order of the SimiÆ into three groups:
(1) Primarii, man together with the other AnthropomorphÆ, or tailless apes; (2) SimiÆ, all the other monkeys; (3) ProsimiÆ, or Lemurs. This arrangement has received strong support from the interesting discovery by Selenka that the peculiar placentation of the human embryo is the same as in the great apes, and different from that of all the other monkeys. Our choice between these different classifications of Primates is best determined by the important thesis of Huxley, in which, in 1863, he carried out a most careful and critical comparison of all the anatomical gradations within this order. In my opinion, this ingenious thesis—which I have called the Huxleyan Law, or the 'Pithecometra-thesis of Huxley'—is of the utmost value. It runs as follows: 'Thus, whatever system of organs be studied, the comparison of their modifications in the ape-series leads to one and the same result—that the structural differences which separate man from the gorilla and the chimpanzee are not so great as those which separate the gorilla from the lower apes.' If we accept the Huxleyan law without prejudice, and apply it to the natural classification of the Primates, we must concede that man's place is within the order of the SimiÆ. On examining this relation with care, and judging with logical persistence, we may even go a step further. Instead of the wider conception of 'SimiÆ,' we must use the restricted term of CatarrhinÆ, and our Pithecometra-thesis has then to be formulated as follows: The comparative anatomy of all organs of the group of Catarrhine SimiÆ leads to the result that the morphological differences between man and the great apes are not so great as are those between the man-like apes and the lowest CatarrhinÆ. In fact, it is very difficult to show why man should not be classed with the large apes in the same zoological family. We all know a man from an ape; but it is quite another thing to find differences which are absolute and not of degree only. Speaking generally, we may say that man alone combines the four following features: (1) Erect walk; (2) extremities differentiated accordingly; (3) articulate speech; (4) higher reasoning power. Speech and reason are obviously relative distinctions only—the direct result of more brains and more brain-power, the so-called mental faculties. The erect walk is not an absolutely distinguishing characteristic: the large apes likewise walk on their feet only, supporting their bodies by touching the ground with the backs of their hands—in fact, with their knuckles—and this is a mode of progression very different from that of the tailed monkeys, which walk upon the palms of their hands. There are, however, two obvious differences in the development of the muscles. In man alone the gastrocnemius and the soleus muscle are thick enough to form the calf of the leg, and the glutÆus maximus is enlarged into the buttocks. A fourth glutÆal muscle occurs occasionally in man, while it is constantly present in apes as the so-called musculus scansorius. Concerning the muscles of the whole body, we cannot do better than quote Testut's summary: 'The mass of recorded observations upon the muscular anomalies in man is so great, and the agreement of many of these with the condition normal in apes is so marked, that the gap which usually separates the muscular system of man from that of the apes appears to be completely bridged over.'
There are, for example, the muscles of the ear. In most people the majority, or even all of them, are no longer movable at will, while in the apes they are still in use. The important point, however, is that these muscles are still present in man, although often in a reduced condition. They are the following: (1) Musculus auricularis anterior or attrahens auris, which is frequently much reduced and no longer reaches the ear at all, being then absolutely useless; (2) Musculus auricularis superior or attollens auris, more constant than the former; (3) Musculus auricularis posterior or retrahens auris, likewise often functional. Occasionally smaller slips differentiated from these three muscles are present, and as so-called intrinsic muscles[Pg 15]
[Pg 16] are restricted to the ear itself; their function is, or was, that of curling up or opening the external ear.
Outlines of the Left Ear of—
1. Lemur macaco; 2. Macacus rhesus, the Rhesus monkey; 3. Cercopithecus, a macaque; 4. human embryo of six months; 5. man, with Darwin's point well retained: the dotted outline is that of the ear of a baboon; 6. orang-utan (after G. Schwalbe):[8] x the original tip of the ear; 7. human ear with the principal muscles.
In connection with the ear, I may touch upon another interesting and most suggestive little feature which is present in many individuals—namely, 'Darwin's point.' This is the last remnant of the original tip of the ear, before the outer, upper, and hinder rim became doubled up or folded in. It is a feature quite useless, and absolutely impossible of interpretation, excepting as the vestige of such previous ancestral conditions as are normal in the monkeys.
In some cases the reduction of muscles has proceeded further in apes than in man—for example, the muscles of the little toe. Another instance is afforded by the coccyx or vestige of the tail; this is still furnished with muscles which are now in man, as well as in the apes, quite useless, and vary considerably with every sign of degeneration, most so in the orang-utan.
Darwin has mentioned the frequent action of the 'snarling muscle,' by which, in sneering, our upper canine teeth are exposed, like those of a dog prepared to fight.
Monkeys and apes possess vocal sacs, especially large in the orang-utan; survivals of them, although no longer used, persist in man in the shape of a pair of small diverticula, the pouches of Morgagni, between the true and the false vocal cords.
'In the native Australians, the dental formula appears least removed from the hypothetical original type, for in it are still found complete rows of splendid teeth, with powerfully-developed canines and molars, the latter being either uniform, or even increasing in size, as we proceed backwards, in such a way that the wisdom tooth is the largest of the series. This is decidedly a pithecoid characteristic which is always found in apes. The upper incisors of the Malay, apart from their prognathous disposition, have occasionally a distinctly pithecoid form, their anterior surface being convex, and their lingual surface slightly concave. The ancestors of Europeans seem to have had the same form of teeth, for the oldest existing fragments of skulls from the Mammoth age (e.g., the jaws from La Naulette, in Belgium) reveal tooth-forms which must be classed with those of the lowest races of to-day.'[9]
Now we are able to apply this fundamental Pithecometra-thesis directly to the classification of the Primates and to the phylogeny of man, which is intimately connected with it, because in this order, as in all the other groups of animals, the natural system is the clear expression of true phylogenetic affinity. Four results follow from our thesis: (1) The Primates, as the highest legion or order of mammals, form one natural, monophyletic group. All the Lemures, SimiÆ, and Homines descend from one common ancestral form, from a hypothetical 'Archiprimas.' (2) The Lemures are the older and lower of the natural groups of the Primates; they stand between the oldest Placentalia (Prochoriata) and the true SimiÆ. (3) All the CatarrhinÆ, or Eastern SimiÆ, form one natural monophyletic group. Their hypothetical common ancestor, the Archipithecus, may have descended directly or indirectly from a branch of the Lemures. (4) Man is descended directly from one series of extinct Catarrhine ancestors. The more recent ancestors of this series were tailless anthropoids (similar to the Anthropopithecus), with five sacral vertebrÆ. The more remote ancestors were tailed Cercopitheci, with three or four sacral vertebrÆ.
These four theses possess, in my opinion, absolute certainty. They are independent of all future anatomical, embryological, and palÆontological discoveries which may possibly throw more light upon the details of our phyletic anthropogenesis.
