The Descent of Man – Day 38 of 151

It is well known that the hair on our arms tends to converge from above and below to a point at the elbow. This curious arrangement, so unlike that in most of the lower mammals, is common to the gorilla, chimpanzee, orang, some species of Hylobates, and even to some few American monkeys. But in Hylobates agilis the hair on the fore-arm is directed downwards or towards the wrist in the ordinary manner; and in H. lar it is nearly erect, with only a very slight forward inclination; so that in this latter species it is in a transitional state. It can hardly be doubted that with most mammals the thickness of the hair on the back and its direction, is adapted to throw off the rain; even the transverse hairs on the fore-legs of a dog may serve for this end when he is coiled up asleep. Mr. Wallace, who has carefully studied the habits of the orang, remarks that the convergence of the hair towards the elbow on the arms of the orang may be explained as serving to throw off the rain, for this animal during rainy weather sits with its arms bent, and with the hands clasped round a branch or over its head. According to Livingstone, the gorilla also “sits in pelting rain with his hands over his head.” (8. Quoted by Reade, ‘The African Sketch Book,’ vol i. 1873, p. 152.) If the above explanation is correct, as seems probable, the direction of the hair on our own arms offers a curious record of our former state; for no one supposes that it is now of any use in throwing off the rain; nor, in our present erect condition, is it properly directed for this purpose.

It would, however, be rash to trust too much to the principle of adaptation in regard to the direction of the hair in man or his early progenitors; for it is impossible to study the figures given by Eschricht of the arrangement of the hair on the human foetus (this being the same as in the adult) and not agree with this excellent observer that other and more complex causes have intervened. The points of convergence seem to stand in some relation to those points in the embryo which are last closed in during development. There appears, also, to exist some relation between the arrangement of the hair on the limbs, and the course of the medullary arteries. (9. On the hair in Hylobates, see ‘Natural History of Mammals,’ by C.L. Martin, 1841, p. 415. Also, Isidore Geoffroy on the American monkeys and other kinds, ‘Hist. Nat. Gen.’ vol. ii. 1859, pp. 216, 243. Eschricht, ibid. s. 46, 55, 61. Owen, ‘Anatomy of Vertebrates,’ vol. iii. p. 619. Wallace, ‘Contributions to the Theory of Natural Selection,’ 1870, p. 344.)

It must not be supposed that the resemblances between man and certain apes in the above and in many other points–such as in having a naked forehead, long tresses on the head, etc.,–are all necessarily the result of unbroken inheritance from a common progenitor, or of subsequent reversion. Many of these resemblances are more probably due to analogous variation, which follows, as I have elsewhere attempted to shew (10. ‘Origin of Species,’ 5th edit. 1869, p.194. ‘The Variation of Animals and Plants under Domestication,’ vol. ii. 1868, p. 348.), from co-descended organisms having a similar constitution, and having been acted on by like causes inducing similar modifications. With respect to the similar direction of the hair on the fore-arms of man and certain monkeys, as this character is common to almost all the anthropomorphous apes, it may probably be attributed to inheritance; but this is not certain, as some very distinct American monkeys are thus characterised.

Although, as we have now seen, man has no just right to form a separate Order for his own reception, he may perhaps claim a distinct Sub-order or Family. Prof. Huxley, in his last work (11. ‘An Introduction to the Classification of Animals,’ 1869, p. 99.), divides the primates into three Sub-orders; namely, the Anthropidae with man alone, the Simiadae including monkeys of all kinds, and the Lemuridae with the diversified genera of lemurs. As far as differences in certain important points of structure are concerned, man may no doubt rightly claim the rank of a Sub-order; and this rank is too low, if we look chiefly to his mental faculties. Nevertheless, from a genealogical point of view it appears that this rank is too high, and that man ought to form merely a Family, or possibly even only a Sub-family. If we imagine three lines of descent proceeding from a common stock, it is quite conceivable that two of them might after the lapse of ages be so slightly changed as still to remain as species of the same genus, whilst the third line might become so greatly modified as to deserve to rank as a distinct Sub-family, Family, or even Order. But in this case it is almost certain that the third line would still retain through inheritance numerous small points of resemblance with the other two. Here, then, would occur the difficulty, at present insoluble, how much weight we ought to assign in our classifications to strongly-marked differences in some few points,–that is, to the amount of modification undergone; and how much to close resemblance in numerous unimportant points, as indicating the lines of descent or genealogy. To attach much weight to the few but strong differences is the most obvious and perhaps the safest course, though it appears more correct to pay great attention to the many small resemblances, as giving a truly natural classification.

In forming a judgment on this head with reference to man, we must glance at the classification of the Simiadae. This family is divided by almost all naturalists into the Catarrhine group, or Old World monkeys, all of which are characterised (as their name expresses) by the peculiar structure of their nostrils, and by having four premolars in each jaw; and into the Platyrrhine group or New World monkeys (including two very distinct sub-groups), all of which are characterised by differently constructed nostrils, and by having six premolars in each jaw. Some other small differences might be mentioned. Now man unquestionably belongs in his dentition, in the structure of his nostrils, and some other respects, to the Catarrhine or Old World division; nor does he resemble the Platyrrhines more closely than the Catarrhines in any characters, excepting in a few of not much importance and apparently of an adaptive nature. It is therefore against all probability that some New World species should have formerly varied and produced a man-like creature, with all the distinctive characters proper to the Old World division; losing at the same time all its own distinctive characters. There can, consequently, hardly be a doubt that man is an off-shoot from the Old World Simian stem; and that under a genealogical point of view he must be classed with the Catarrhine division. (12. This is nearly the same classification as that provisionally adopted by Mr. St. George Mivart, (‘Transactions, Philosophical Society,” 1867, p. 300), who, after separating the Lemuridae, divides the remainder of the Primates into the Hominidae, the Simiadae which answer to the Catarrhines, the Cebidae, and the Hapalidae,–these two latter groups answering to the Platyrrhines. Mr. Mivart still abides by the same view; see ‘Nature,’ 1871, p. 481.)

The anthropomorphous apes, namely the gorilla, chimpanzee, orang, and hylobates, are by most naturalists separated from the other Old World monkeys, as a distinct sub-group. I am aware that Gratiolet, relying on the structure of the brain, does not admit the existence of this sub-group, and no doubt it is a broken one. Thus the orang, as Mr. St. G. Mivart remarks, “is one of the most peculiar and aberrant forms to be found in the Order.” (13. ‘Transactions, Zoolog. Soc.’ vol. vi. 1867, p. 214.) The remaining non-anthropomorphous Old World monkeys, are again divided by some naturalists into two or three smaller sub-groups; the genus Semnopithecus, with its peculiar sacculated stomach, being the type of one sub-group. But it appears from M. Gaudry’s wonderful discoveries in Attica, that during the Miocene period a form existed there, which connected Semnopithecus and Macacus; and this probably illustrates the manner in which the other and higher groups were once blended together.

If the anthropomorphous apes be admitted to form a natural sub-group, then as man agrees with them, not only in all those characters which he possesses in common with the whole Catarrhine group, but in other peculiar characters, such as the absence of a tail and of callosities, and in general appearance, we may infer that some ancient member of the anthropomorphous sub-group gave birth to man. It is not probable that, through the law of analogous variation, a member of one of the other lower sub-groups should have given rise to a man-like creature, resembling the higher anthropomorphous apes in so many respects. No doubt man, in comparison with most of his allies, has undergone an extraordinary amount of modification, chiefly in consequence of the great development of his brain and his erect position; nevertheless, we should bear in mind that he “is but one of several exceptional forms of Primates.” (14. Mr. St. G. Mivart, ‘Transactions of the Philosophical Society,’ 1867, p. 410.)

Every naturalist, who believes in the principle of evolution, will grant that the two main divisions of the Simiadae, namely the Catarrhine and Platyrrhine monkeys, with their sub-groups, have all proceeded from some one extremely ancient progenitor. The early descendants of this progenitor, before they had diverged to any considerable extent from each other, would still have formed a single natural group; but some of the species or incipient genera would have already begun to indicate by their diverging characters the future distinctive marks of the Catarrhine and Platyrrhine divisions. Hence the members of this supposed ancient group would not have been so uniform in their dentition, or in the structure of their nostrils, as are the existing Catarrhine monkeys in one way and the Platyrrhines in another way, but would have resembled in this respect the allied Lemuridae, which differ greatly from each other in the form of their muzzles (15. Messrs. Murie and Mivart on the Lemuroidea, ‘Transactions, Zoological Society,’ vol. vii, 1869, p. 5.), and to an extraordinary degree in their dentition.

The Catarrhine and Platyrrhine monkeys agree in a multitude of characters, as is shewn by their unquestionably belonging to one and the same Order. The many characters which they possess in common can hardly have been independently acquired by so many distinct species; so that these characters must have been inherited. But a naturalist would undoubtedly have ranked as an ape or a monkey, an ancient form which possessed many characters common to the Catarrhine and Platyrrhine monkeys, other characters in an intermediate condition, and some few, perhaps, distinct from those now found in either group. And as man from a genealogical point of view belongs to the Catarrhine or Old World stock, we must conclude, however much the conclusion may revolt our pride, that our early progenitors would have been properly thus designated. (16. Haeckel has come to this same conclusion. See ‘Uber die Entstehung des Menschengeschlechts,’ in Virchow’s ‘Sammlung. gemein. wissen. Vortrage,’ 1868, s. 61. Also his ‘Naturliche Schopfungsgeschicte,’ 1868, in which he gives in detail his views on the genealogy of man.) But we must not fall into the error of supposing that the early progenitor of the whole Simian stock, including man, was identical with, or even closely resembled, any existing ape or monkey.

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