Functional Affinities of Man, Monkeys, and Apes

A Study of the Bearings of Physiology and Behaviour on the Taxonomy and Phylogeny of Lemurs, Monkeys, Apes, and Man

S. Zuckerman

• Publisher: Routledge
• Year: 2019
• Edition: 1
• eBook ISBN:
  9781000063707
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Originally published in 1933 Functional Affinities of Man, Monkeys and Apes gives a taxonomic and phylogenetic survey and the findings of diverse experimental investigations of lemurs, monkeys, and apes. The book discusses the inter-relationships of different Primates and emphasizes seldom-used approaches to the question of primate phylogeny. The book attempts to show how little they have been systematically tried, and argues for a regard to the proper place of functional investigations in the study of the classification and evolution of Primates. This book will be of interest to anthropologists, scientists and historians alike.

CHAPTER I

PECULIARITIES OF CLASSIFICATION IN THE ORDER PRIMATES

SCIENTIFIC thought becomes increasingly difficult the less its material is amenable to quantitative treatment and the more it is related to deeply rooted emotional attitudes. This is well recognized in questions of human origins, and students in this field usually realize that they must subdue what anthropocentric interest they naturally have in their subject if they wish their conclusions to have a sound and unbiassed foundation. Thus Darwin (1871), a prominent exponent of objectivity, writes that the descent of man should be discussed in the same way as that of any animal. What, for instance, are man’s relations to other vertebrates? Does he vary in bodily structure and mental faculties? If he does, are his variations “governed by the same general laws, as in the case of other organisms?” Has he given rise to “varieties and sub-races, differing but slightly from each other, or to races differing so much that they must be classed as doubtful species?”

A complete answer to questions as broad as these demands extremely wide study, particularly of human genetics, and far wider, indeed, than Darwin himself probably realized. Until such studies have been made, views on human descent must continue to have a more modest, but nevertheless not altogether insufficient basis in the findings of palæontology, and in the comparison of man with other animals (particularly with the sub-human Primates) in regard to structure, embryology and growth, physiology, susceptibility to disease (including parasitic infections) and behaviour. Some of these fields of relationship are, unfortunately, not so widely known or explored as are others, current views on the phylogeny of the Primates being largely based on the conclusions of comparative anatomy and the relatively slight evidence provided by palæontology. The physiological evidence, with the exception of the blood reactions, is seldom considered. The same may be said about the facts relating to disease and behaviour.

These omissions in the discussion of the classification and phylogeny of the Primates are unnecessary. Partly they are the results of too narrow a specialization in primate studies, for natural interest in human structure, and the difficulties of observing living animals, have combined to lay far too heavy an emphasis, in phylogenetic discussion, on comparative morphology. Partly they are also due to the absence of any effective co-operation between the systematist on the one hand, and the physiologist, or biochemist, or helminthologist on the other. Although the professional systematist is trying to express animal relationships as correctly as possible, he is generally compelled to draw his conclusions from the evidence of morphology alone. Even should he possess the necessary training in other fields, it is unlikely that he would have the time to examine the journals of physiology, or biochemistry, or experimental medicine, in search of facts which, though primarily of interest to students in these fields from an entirely different point of view, may nevertheless have bearing on the taxonomic questions he is studying. As a result of this, the real meaning of taxonomy—the science of the laws of arrangement or classification—has in some quarters been narrowed to mean classification on the basis of a few arbitrary structural characters. If this restricted interpretation is disregarded, little reflection is needed in order to realize that the consideration of phylogeny can have a much firmer taxonomic foundation in the order Primates than in other groups of mammals where, owing to lack of knowledge, it is usually of necessity limited to palæontology and more superficial anatomy.

This fact helps to explain why, apart from anthropocentric interest, the systematics of the order of animals to which man belongs has so peculiar an appeal. It also suggests why it is so controversial a subject. The characters by which taxonomic and phylogenetic relationships are traced may or may not be conspicuously related to one another; but, whether they are correlated or not, the more they are multiplied, the more scope do they provide for argument, and the greater is the resulting confusion.

There are, however, other explanations of the endless scientific controversies about human descent. For example, the phylogeny of the Primates is mostly a preoccupation of anatomists with a medical training, whereas animal phylogeny as a general study is a problem of professional zoologists. Naturally, therefore, a profound difference in point of view often exists between the human anatomists and the zoologists who take up the studies of the phylogeny of man and the classification of the Primates. The zoologist is warned against constituting a species with few individuals and on a small number of characters. Moreover, his greater experience of animal forms gives him a better opportunity for becoming the competent systematist by whose judgment a species can be defined—to use a “definition” of species that seems to be greatly in vogue at the present time.* He realizes, at least in theory, that if one group of animals differs from another in the absence of certain characters and the greater expression of others common to both groups, the true value of the difference can be assessed only statistically. The anatomist, on the other hand, usually approaches the problem from a totally different angle, long acquaintance with the human form giving him a somewhat unwarranted confidence in the systematics of the Hominidæ. When the anthropometrist (see Morant, 1926 and 1930) tells him that skulls which he has regarded as distinct types are, from the point of view of measurement, a sample from a homogeneous population, he may refuse to accept such a view. He may affirm (see Keith, 1931) that in his craniological studies he is guided by anatomical characters whose value can be assessed, for purposes of race identification, by the unaided eye—apparently forgetting for the moment that measurement can only increase the power of observation, and that he continually uses it where he is able. As I have tried to show elsewhere (Zuckerman, 1933), the main result of this arbitrary “anatomical” treatment of the bony remains of archaic men is that to-day we accept an altogether irrational classification for the family Hominidæ.

But in his acclamation of this peculiar point of view, the anatomist may partly be labouring under the influence of a widely spread impulse to hail new observations m the field of primate studies as unparalleled and unique in their significance, irrespective of their exact relationship to pre-existing knowledge. It is an attitude sometimes demonstrated also by palæontologists, and one of its effects is chaos in the classification of another family of Primates, the Pongidæ (apes). The somewhat arbitrary systematic reference of fossil Primates may be exemplified by the following instance. At a recent meeting of the Anatomical Society of Great Britain and Ireland, Hopwood, who speaks with authority as a palæontologist, exhibited a maxilla, together with some teeth, found in a Central African Miocene deposit (believed to be Lower Miocene). He pointed out that the teeth are of the Dryopithecus pattern, and that they closely conform to the classical description, given by Gregory (1916), of the corresponding chimpanzee teeth. This he interpreted as indicating that the fossils represent an ancestral form of chimpanzee, but rather than refer them to the genus Pan (chimpanzee) or to the genus Dryopithecus, he suggested the creation of a new genus, Proconsul (see Hopwood, 1933).

* A similar “definition” occurs in Darwin’s Origin of Species.

Arbitrary though the creation of new genera and species may be, there can be little justification for this proposal. None can be found on the score of the age of the teeth, for there is no obvious reason why the same genus should not have persisted more or less unchanged through several geological epochs. An actual instance of this is provided by the Lamp shells of the Cambrian, which palæontologists are quite satisfied to call by the generic name (Lingula) that is used for existing shells of the same kind. Thus unless justification for the creation of new genera of apes or man is to be sought on the score of convenience of reference, a skull bearing the characters of Homo sapiens should be classified as Homo sapiens, even if it were recovered from an Oligocene deposit. If the teeth described by Hopwood are as much like those of a chimpanzee as he stated, there is no reason why they should not be included among the chimpanzees in the genus Pan. Moreover, precedent for more conservative treatment of the teeth of fossil apes is well provided by the genus Dryopithecus, which consists of more than six species, the teeth of which would seem to differ far more amongst themselves than the teeth of Proconsul differ from the corresponding teeth of the chimpanzee.* Fortunately for those who wish to see a less sensational classification and phylogeny of Primates than is usual, the danger of creating genera on the strength of single teeth is sometimes strikingly apparent, as the instance of Hesperopithecus shows, even though such procedure may occasionally be brilliantly justified, as in the case of Sinanthropus.

Proconsul is not a solitary exception to an otherwise well-balanced discipline of classification and phylogenetic study. Indeed, cases of a similar kind are so numerous as to force the conviction that primate material has a peculiar power of overweighting the conclusions of its students. This is unfortunate. General interest demands the constant re-telling of the tale of human evolution. In the circumstances this tale becomes either distorted or more dogmatic than the evidence warrants. Whether we like it or not, almost every fossil primate form, at least in the first excitement following its discovery, is given some special significance in the story of man’s descent (e.g. the Taung’s fossil). Moreover, the importance, whether it be real or unreal, attached to the question of man’s evolution, prevents any separation of primate classification from primate phylogeny. The two are firmly knit together. It is essential, therefore, that the genera within a single family such as the Pongidæ, or the species within a single genus, should be at least approximately equivalent in their taxonomic differentiation, if the theoretical condition is to be fulfilled that the sub-groups of a given genus or family can be regarded as more closely related to each other by descent than they are to the members of other genera or families. Biassed speculation on the descent of the Primates only too easily prevents this, and a classification once made soon bears fruit. New students will accept without question a classification in which the relative difference between two genera of a family may actually be less than that between two species of a single genus, and will automatically draw distorted phylogenetic conclusions. If only to prevent this, a strong case could be made for the delimitation of taxonomic groups on a basis of a sliding scale of artificial units.

* See Gregory and Hellman, 1926.

CHAPTER II

THE DYNAMIC BASIS OF CLASSIFICATION AND EVOLUTIONARY THEORY

THE explanation of the processes of variation and the scientific assessment of the concept of selection are the critical points on which the validity of theories of evolution ultimately depend. Neither variation nor selection can be adequately studied except by experiment and statistical analysis, and it is undoubtedly true that such studies as experimental genetics have vitalized the evolutionary hypothesis by providing it with a physiology. Their influence can be recognized in all biological fields in which the evolutionary concept has to be considered, and the effect they have had on such a subject as palæontology is one of the more remarkable features of twentieth-century biology. Even palæontologists who believe that new characters arise through the adaptation of old ones to changed conditions and function, realize that their views require further examination in the light of modern experimental genetics.

The experimental method can also find a field of application in taxonomy, and in phylogeny—the dynamic interpretation which the evolutionary concept allows to be made of the ideal classification or “natural system” provided by taxonomy. An ideal classification would be based upon a complete analysis of the resemblances and differences of all the “characters” that could be isolated from the organisms under consideration. And in this connection, characters, to quote Robson (1928), “include every structure and property of the animal or plant, whether they be organs, cytological structure, physiological activities, habits or ecological relationships.” The obvious difficulties that prevent the realization of such an ideal classification also constitute the obstacles which serve to separate taxonomy from phylogeny. If, however, these fields of study are to be adequate, they must always tend towards each other, however far circumstances may keep them apart. A restriction of outlook in either the theory or the practice of taxonomy may easily lead to irrational conclusions. Thus in discussing the descent of man, which more widely is the problem of the phylogeny of the Primates, it may be misleading to refer, as Gregory does (1922), to “the taxonomic, palæontological and anatomical evidence”, as though there existed a distinction between the taxonomic and anatomical evidence bearing on the problem, and as though the one kind of evidence supported the other. It is even more dangerous to base a classification on too few characters, or on characters weighted for arbitrary reasons. As Crow (1926) has pointed out, “each character, as it is observed in nature, must be accorded equal weight; there can be no evaluation of characters on the nature of the characters themselves.’’ Such value ultimately depends upon the positions taxonomic characters assume in a scale showing their distributions within the group of organisms in which they are manifested. In practice, of course, organisms are classified according to purely superficial resemblances, but as Crow has written, it has been established in many cases that if a number of forms are arranged “in respect to the degree of development of one character they are also ipso facto arranged in a series in respect to other characters.” In other words, characters are often correlated with one another, and we may, with justification, refer to the “character complex” of a species. Unfortunately the many cases which do not show such correlation of characters provide only too ready an explanation for the controversies of systematic biology.

A rational definition of “characters” such as that provided by Robson does away with any necessity to excuse the introduction of data relating to function into discussions of taxonomy or phylogeny.* If precedent for such a step be required, it is only necessary to remember the dynamic basis Cuvier gave to his “Règne Animal” (1817), or the more recent welcome given by systematists to Nuttall’s classic researches (1904) on the serum precipitin reaction. Unfortunately, however, experimental biology is a young subject, and knowledge of the extent to which physiological differentiation occurs in the living world is very limited. It is unusual for enough facts about function to be available, as they are in the case of the Primates, to justify an attempt to trace their phyletic ramifications within the narrow boundaries of a single order of animals.

* The question is discussed at length by Robson. Crow’s (1926) view “that physiology, as ordinarily understood, cannot affect the findings of phylogeny” is based on the assumption that physiology is only concerned with “explaining the life processes” in terms of physics and chemistry. The contents of any physiological journal, however, show that a large part of its function is also to discover and describe these processes, which vary considerably from one living form to another. Like morphological characters they can therefore be used in taxonomy.

Apart from its use in providing taxonomic characters, the experimental method has other obvious and important functions to fulfil in taxonomy. Thus it should be used where possible to test the value of those single morphological characters for which special virtue is claimed in classification. A possible use, for example, is provided by the problem recently raised by Regan (1930). Regan has proposed a new and very unorthodox phyletic classification of the Primates, based, almost entirely, on the straight or wavy character of the enamel prisms of the teeth. He has not, however, provided any reason to explain why this character, which was described by Carter in 1922, should be regarded as of greater consequence than the correlated series of morphological features on whose distribution depends the orthodox view. Because of this omission it might quite reasonably be doubted whether his suggestion is sufficiently valuable to demand examination. An altogether different light would be thrown on the character, however, if his view were supported by an experimental study showing the factors which determine whether enamel prisms should have straight or wavy edges. It is of course possible that “straight” and “wavy”, in regard to enamel prisms, are discontinuous characters, and that adequate statistical study will establish Carter’s conclusions that they are differentiated as such among the different groups of Primates. If this were proved, enamel prisms would become a genetically distinct character of taxonomic value, and ipso facto would demand equal consideration with other such characters. As yet, however, it has not been shown whether or not their shape is affected by ontogenetic age, or by the mineral and vitamin content of the diet. Until this is done, there is no good reason for investing them with any special phyletic significance even though they suggest a novel classification of the Primates.

There are other individual characters used in taxonomy which could first be investigated experimentally, but only one more—coat colour—need be mentioned here. This character is constantly referred to in primate systematics, and if the ideal were possible, its physiology and genetics would doubtlessly be considered before it is used to differentiate species. If experimental knowledge of this kind were available, it would meet such difficulties a...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Original Title Page
• Original Copyright Page
• Dedication
• Contents
• List of Illustrations
• Foreword
• Chapter I. Peculiarities of Classification in the Order Primates
• Chapter II. The Dynamic Basis of Classification and Evolutionary Theory
• Chapter III. The Subdivision of the Order Primates
• Chapter IV. The Differentiation of the Mechanisms of Reproduction
• Chapter V. The Differentiation of Blood Reactions
• Chapter VI. The Differentiation of Receptor Organs and Their Functions
• Chapter VII. The Differentiation of Behaviour Patterns
• Chapter VIII. The Diseases and Parasites of the Primates
• Chapter IX. Hybridization, Affinity, and Divergence
• Chapter X. The Psychological Measure of Intelligence in Primates
• Chapter XI. The Brain as a Measure of Intelligence in the Sub-Order Pithecoidea
• Chapter XII. The Phylogenetic Implications of Cortical Physiology
• Chapter XIII. The Evolution of Primate Behaviour
• Chapter XIV. Functional Differentiation in Relation to the Evidence of Morphology and Palæontology
• Bibliography
• Index