It is now generally admitted that this problem is the most important of all biological questions. Huxley was right when in 1863 he called it the question of questions for mankind. The problem which underlies all others, and is more deeply interesting than any other, is as to the place which man occupies in nature and his relations to the universe of things. 'Whence our race has come; what are the limits of our power over nature, and of nature's power over us; to what goal are we tending—these are the problems which present themselves anew and with undiminished interest to every man born into the world.' This impressive view was explained by Huxley thirty-five years ago in his three celebrated essays on 'Evidence as to Man's Place in Nature.' The first is entitled 'On the Natural History of the Man-like Apes'; the second, 'On the Relations of Man to the Lower Animals'; the third, 'On some Fossil Remains of Man.' Darwin himself felt the burden of these problems as much as Huxley; but in his chief work, 'On the Origin of Species,' in 1859, he had purposely only just touched them, suggesting that the theory of descent would shed light upon the origin of man and his history. Twelve years later, in his celebrated work on 'The Descent of Man, and Selection in Relation to Sex,' Darwin discussed fully and ingeniously all the different sides of this 'question of questions' from the morphological, historical, physiological, and psychological points of view. As early as 1866 I myself had applied in the Generelle Morphologie der Organismen the theory of transformism to anthropology, and had shown that the fundamental law of biogeny claims the same value for man as for all the other animals. The intimate causal connection between ontogeny and phylogeny, between the development of the individual and the history of its ancestors, enables us to gain a safe and certain knowledge of our ancestral series. I had at that time distinguished in this series ten chief degrees of vertebrate organization. I attributed the highest importance to the logical connection of anthropogeny with transformism. If the latter be true, the truth of the former is absolute. 'Our theory that man is descended from lower vertebrates, and immediately from apes or primates, is a case of special deduction which follows with absolute certainty from the general induction of the theory of descent.' The full proof and detailed explanation of this view was afterwards given in my 'History of Natural Creation,' and especially in my 'Anthropogeny.'[3] Lastly, it has received an ample scientific and critical foundation in the third part of my 'Systematic Phylogeny.'[3]
During the forty years which have elapsed since Darwin's first publication of his theories an enormous literature, discussing the general problems of transformism as well as its special application to man, has been published. In spite of the wide divergence of the different views, all agree in one main point: the natural development of man cannot be separated from general transformism. There are only two possibilities. Either all the various species of animals and plants have been created independently by supernatural forces (and in this case the creation of man also is a miracle); or the species have been produced in a natural way by transmutation, by adaptation and progressive heredity (and in this case man also is descended from other vertebrates, and immediately from a series of primates). We are absolutely convinced that only the latter theory is fully scientific. To prove its truth, we have to examine critically the strength of the different arguments claimed for it.
I.
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 are restricted to the ear itself; their function is, or was, that of curling up or opening the external ear.
