1902 Encyclopedia > Ape > The Zoological Position and Affinities of the Apes

(Part 30)


(D) Special Topics (cont.)

The Zoological Position and Affinities of the Apes

By universal consent apes are placed in the highest rank of al brutes, and, excepting man, are generally taken to be the most perfect animals of the mammalian class. It may be questioned, however, whether, if the animal man had never existed, this place would be assigned them by any observing intelligence. The half-apes, or lemurs, commonly placed in the same order with them, are certainly inferior mammals; and it might be contended that the perfection of the mammalian type is rather to be found in the Felidae (or cat family), by reasoning analogous to that by which it might also be contended that birds(with their differentiated limbs, perfect circulating and respiratory systems, acute sense organs, complex instincts, and teachableness) are really the highest of all vertebrate animals, and represent the vertebrate type of structure carried to the highest degree of perfection yet attained.

The question as to which animals are most nearly allied to apes is one by no means easy to answer. Leaving man aside (whose close anatomical resemblance to apes is to obvious), it is at present extremely difficult to say what are the apes' true zoological affinities. It is to be hoped that future palaeontological researchers may afford us materials for tracing these out; but at present a chasm separates the apes from every other group of animals. The half-apes, or lemurs, were generally considered to lead down from the apes towards the insectivora, and thence to the implacental mammals, but the differences between the apes and lemurs are so many and great, that it cannot be considered otherwise than in the highest degree improbable that (on the evolution hypothesis) they took origin from any common root-form that was not equally the progenitor of other mammalian orders.

But if the apes cannot be considered to show evidence of genetic affinity with any other mammalian order, to they constitute so homogeneous a group as to suggest the former existence of one ancient root-form common to them all?

To this question it may be answered that the differences between the Simiadae and Cebidae are such as to render it doubtful whether they may not have had respectively quite different origins, and whether their resemblances may not have been superinduced by similarity of needs and conditions. The differences referred to are as to -- (1), dentition; (2), nasal septum; (3), tail -- the Cebidae showing a tendency to a curled tail-end, while the Simiadae never manifest any such tendency; (4), cheek pouches; (5), ischiatic callosities; (6), general form and habit of body; (7), opposability of the thumb; (8), bony meatus auditorius externus.

All these characters, taken together, seem to make it probable that the Cebidae and Simiadae are not diverging offshoots from some common ape parent, but that they have arisen in an independence as complete as that between the origin of either of them and the origin of the lemuroids or carnivores. Possibly further discoveries in the Miocene deposits of North America will reveal to us transitional forms between the Old and the New World apes, but the existence of such forms cannot certainly as yet be affirmed.

It may be asked, however, Can the genera, which possess so many points in common as Cebus and Cercopithecus, have come to resemble each other independently?

To this it may be replied, that the number of similarities of structure which must have had an independent origin is so great that it is difficult to see why those of two genera named may not also have had such an origin. As examples of such similarities of independent origin, the following structures may be referred to:-- The bony covering of the temporal fossa in Chelonia, Pelobates, and Lophiomys; the compound tooth structure of Orycteropus and Myliobatis; the coexistence of a certain form of dentition with a salutatory habit in Macropus and Macroscelides; the presence of but eight carpal bones in Troglodytes and Indris; the course of the vertebral artery in Auchenia and Myrmecophaga; the flying membrane in certain squirrels and phalangers; the canines and premolars of Canis and Thylacinus; the grinders of Peromeles and Urotrichus; the external form and habit of body of Mus, Sorex, and Antechinus; and the peculiar dorsal shields in tortoises and certain frogs.

But if some naturalists are disposed to admit that the common origin of the Cebidae and Simiadae may be very doubtful, can they be even sure of that of Cercopithecus and Hylobates? It has been recently suggested, that the Artiodactyla and the Perissodactyla (the even and the odd-toed ungulates) may be genetically independent (their common characters being merely adaptive, functional ones), as also with the Balaenoidea (whales), and Delphinoidea (dolphins).

The response of organization to need being such as it is (structure and function manifesting themselves so simultaneously), the discrimination between genetic and adaptive characters must always be a work of extreme delicacy. In the presence of the various genealogical trees of animal descent which have been so hastily put forward of late, a judicious skepticism seems the attitude best warranted by the evidence yet obtained. If so many similar forms have arisen in mutual independence, then the affinities of the animal kingdom, or even of the mammalian class, can never be represented by the symbol of a tree. Rather, we should conceive the existence of a grove of trees, closely approximated, greatly differing in age and size, with their branches interlaced in a most complex entanglement. The great group of apes is composed of two such branches; but their relations one to another, to the other branches which represent mammalian groups, and to the trunks from which such branches diverse, are problems still awaiting solution.

There can, however, be no doubt that the Simiadae and Cebidae together form a most natural group, and are closely allied with man in structure. Moreover, as man is the highest animal, and, zoologically considered, differs less from even the lowest ape than such ape differs from any other animal, man and apes must be placed together in one order, which, may well bear its primitive Linnean name, "Primates." Whether any other animals (and, if any, what) should also be included in this order, are questions for the consideration of which the reader is referred to the heading MAMMALIA.

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