Popular Science Monthly/Volume 33/October 1888/The Last Stages in the Genealogy of Man I
|THE LAST STAGES IN THE GENEALOGY OF MAN.|||
OUR lectures hitherto have shown us that science has not yet succeeded in casting a clear light on the exact connections of the placental mammalia, and that it is still ignorant of the precise ways, direct or indirect, by which the present orders and families have been derived. Haeckel's genealogy has been the point of departure for numerous essays, which have rendered immense services; but, as the author himself declares, it is only a first sketch, and will have to be revised hereafter. It has been shown by our lectures that the present orders, families, and genera are the product of a long evolution and successive transformations, and did not exist when the first placental mammalia appeared, and when the first feebly determinative evolutionary movement of differentiation and reduplication of types, which led to existing forms, was manifested in the marsupials. It is also shown that the progressive passage from the marsupial fauna of that time to the existing fauna did not take place by a single series of species for each order, family, or genus, but in all the cases in which science is in possession of sufficient documents, by multiple series, anastomosing, intercrossing one another, and often constituting an inextricable network.
Here and there the advance seems to have been more direct, as in the ungulates, the carnivores, the, and the pinnipeds or aquatic carnivora, while in other orders, such as the insectivora and the rodents, it seems to have been in an exceedingly complicated way. That branch which, according to Haeckel, leads to man, is the one that interests us most. Let us consider, then, the station which succeeds that of the marsupials, the eighteenth from the moneres in Haeckel's genealogy, the lemurs.
The lemurs have been ranked among the quadrumana by Geoffroy Saint-Hilaire, Cuvier, De Blainville, Duvernoy and Milne-Edwards—that is, separated from man; and among the primates, or in the same order with man, by Linnæus, Lesson, Huxley, and Broca. Vogt and Haeckel call them prosimians, the Germans half-apes, and the French sometimes false apes. The dominant question in our investigation is, therefore, where they belong: should they be called prosimians, or should they figure among the primates?
The primates might be well defined by saying that they are placentary mammalia, non-aquatic—which excludes the cetaceans, sirens, and pinnipeds; without hoofs—which excludes the ungulates and proboscidians; having three kinds of teeth—which removes the rodents and edentata; and having molars neither with cutting blades nor with sharp, conical points—which excludes the carnivora and insectivora. They have no absolutely peculiar characters in common, naturalists not regarding the type of the cerebral circumvolutions. They have a discoidal placenta, and a uterus with a cavity not two-horned; theor bats have likewise the third characteristic. They have two pectoral mammæ; but so have the bats and the lamantins.
The teeth vary among them as to number, form, and permanence. They appear more specialized, brought nearer to one another, and more fixed in their general form, as we ascend toward man. There are four stages in the last category—the lemurs, the monkeys of the old continent, the monkeys of the new continent, and man.
The replacement of claws by nails forms one of the most important characteristics of the primates. Claws are designed and formed for attack and defense; the hoofs of the ungulates form hard soles for the feet, protecting them from contact with the ground and facilitating the march of the animal; while the nails are so shaped as to be adapted to the purpose of prehension. This adaptation is more or less perfect, and extends to more or fewer fingers among the primates, permitting another division into the perfect primates, like man and all the monkeys but one group, and imperfect primates. Another adaptive characteristic, the corollary of the nails, is the well-developed thumb, removed from the other fingers, and opposable to them. More completely than they, it also indicates an organ made to clasp, to seize. The primates may also be divided by this feature into three groups Man, with whom the thumb is opposable only on the fore-limbs the monkeys, with which it is opposable on all four of the limbs and the imperfect primates, with which the adaptation is less exact or less marked on the hinder than on the fore limbs. Other characteristics, usually graduated in the ascending series of the primates, might be mentioned; but these are enough for our purpose.
To regard the primates in this way is a little to prejudge the solution we are seeking. From the instant we suppose a progressive development of characteristics in the series and divide the primates into superior, medium, and inferior, we are tempted to be indulgent in respect to characteristics which may be little accentuated or wanting in the last. From this, to assuming that the lower primates are simply the beginning of the series, the transition from the other orders to that of the primates, is but a step. The lemurs furnish most of the imperfect primates of which we have spoken. They included or include three groups of animals—the Galeopitheci, the Cheiromys, and the lemurs proper.
The Galeopitheci, or flying-cats, inhabit the Sunda Islands, the Moluccas, and the Philippine Islands. They furnish one of the examples of the difficulty of placing in our classification certain groups qualified with paradoxical characteristics, for the reason that they are transitional groups having some right to be put in several.
The Cheiromys include only one genus, the aye-aye of Madagascar. It resembles the squirrel, but has features also of the ape and the lemur. By dentition it is an insectivore or lemur in infancy and a rodent in adult age. It is evidently a primate at the start, but a species hesitating whether it shall continue a primate or become a rodent.
The lemurs proper are divided into the fossil and the recent. The former appear in the Eocene when there existed parallel with them the marsupials in a declining stage and the first placental mammals—the carnivora, rodents, ungulates, and the insectivora. Europe has furnished five genera of them and America more, the most important among them being the Anaptomorphus, from which Mr. Cope makes man a direct derivative. The recent species are distributed in three geographical groups, the first and most numerous being confined to the island of Madagascar, the second to that island and Africa south of the Sahara, and the third living in the island of Ceylon, the Malacca Peninsula, the Moluccas, and the Philippines, or the regions which Haeckel supposes to constitute the remains of the vast southern continent which he calls lemurian.
The lemurs are tree-dwelling and nocturnal animals. They have four opposable thumbs with the exception of the tarsier, which has only the hind thumbs opposable. All their fingers, as a rule, have nails except the hind forefinger, which has a claw, or in the loris, the fore little finger; but the nails are all badly shaped and seem transitional from claws. A general formula can not be framed for the teeth. The number varies from thirty to thirty-six.
All these facts tend to establish that the lemurs have not a fixed, homogeneous type, but that they constitute a transitional group from animals with claws to animals with nails. They may consequently be regarded as the first or perhaps the second stage (regarding the Cheiromys as the first) toward the better characterized monkeys; but serious objections are brought against this view. One was based by M. Broca on certain features of the placenta, indicating a violent separation in this important characteristic of the lemurs from the other primates. M. Vogt, while not attaching so great importance to this feature as M. Broca, brought other objections, based on diversities in the formation and connections of the jaw-bones, the structure of the orbits, the position of the os lacrymalis, the bare cerebellum of the lemurs, the shape of the uterus, the presence of inguinal mammæ in addition to the pectoral mammæ, and other points. Hence he concluded that there was no relation between the prosimians and the apes, and consequently none with man; and that, except the opposable thumbs, which occur also with the marsupials, the prosimians have no anatomical character in common with' the monkeys. "Therefore it would be derogatory to all the principles of positive science to rank the prosimians among the probable ancestors of the human race." These objections are certainly important from the morphological point of view, but they do not oblige us to reject the lemurs from the order of primates. None of these divergent characteristics are contradictory of the idea that they are the first draught, the beginning, of the latter order. The characteristics drawn from the nails and the opposable thumbs press the others out of the view of the general idea which has directed the choice of the word primates.
The lemurs are the lowest family in the order of primates, and are further removed from the other families than the latter are from one another. The distance from the anthropoids to man is quite as great, as I have demonstrated in previous lectures, on the evidence of the volume of the brain and the cranial characters which proceed from it; and yet I class man among the primates. In strictness we might detach the lemurs and make a special order of them, the genealogical relation of which with the monkeys would not be thus prejudiced; but then we should be obliged to do the same with man. M. Vogt is, nevertheless, not consistent, and retains the word prosimians as the synonym of lemurs.
I have already insisted, in previous lectures, upon the relations of the lemurs with the marsupials, and more particularly with the phalangers. The insectivora come next in order. All authors, from Cuvier to M. Vogt, have mentioned the resemblance between the teeth of lemurs and those of insect-eaters. Their teeth, says Cuvier, arranging his orders downward, from man to the lower mammals, "begin to exhibit sharp tubercles gearing into one another as in the insectivora." "The Galegos," we find a little further on, "have the insectivorous teeth and regimen of the other lemurs. M. Vogt says that the dentition of the tarsiers is like that of the insectivora; and Prof. Huxley observes that the lobes of the molars are habitually very far in front, as in the insectivora. Gratiolet classed the lemurs with the insectivora. Derivation from the insectivora is, however, in no way contradictory with descent from the marsupials. The primitive type of the latter was insectivorous in the Triassic and Jurassic epochs.
The last relation to be considered is that with the ungulates, concerning which we have the observation of M. Albert Gaudry. "I have asked myself," he says, in his "Tertiary Fossils," "if the lemurs had not a community of origin with many of the extinct pachyderms." The resemblances between recent lemurs and the ungulates, pointed out by MM. Alphonse Milne-Edwards and Grandidier in their great work on Madagascar, lend credibility to this opinion. Two genera are conformed to the idea: Adapis, the Parisian species of which, derived from the gypsums of the Upper Eocene of Montmartre, was classed by Cuvier among the pachyderms, but appears, judging by the teeth, the skull, and parts of the limbs, to be only a lemur; and the Aplelotherium, classed by Gervais also with the pachyderms, and now recognized as a lemur. The resemblance occurs among the Eocene species of the stock of recent perissodactyli, such as the Hyracotherium, the Lophiotherium, and the Pachynolophus.
Mr. Cope has also discovered several species of Adapis in the United States, and confirms these resemblances. It is, however, proper to remark that the genealogy leading to man is not in question in this matter. Mr. Cope divides the American fossil lemurs into three families: the Anaptomorphs, which lead, by two branches, one to the monkeys and the other to man; the Mixodectines, the outcome of which I do not know; and the Adapides, which lead to the ungulates. The branch of the Adapis is, therefore, according to Mr. Cope, foreign to the branch leading to man.
We shall shortly now abandon the eighteenth stage or the lemurs of Haeckel, to pass to the nineteenth, that of the catarrhinian apes, or rather to the monkeys as a whole.
The further I go, the more I am convinced that the anthropoids should be joined with the monkeys recognized by all under that name, and that they are only the highest family of them; and the more I am persuaded that they should be separated from man, looking at the matter from a morphological point of view, further than is admitted in a certain school; for the physiological or intellectual point of view is not for an instant discussable. The principal classifications of the primates are as follow:
Cuvier, two groups, man and the monkeys, the latter, under the name of quadrumana, being divided into apes, lemurs, and ouistitis, the first including what are called great apes or anthropoids.
Broca, in his last classification, which is only a variant of that of Linnæus—two groups: man and the anthropoids together; the monkeys, including those of the old continent or the pithecans, and those of the new continent or the cebians.
Huxley, in his last classification—three groups: man, the monkeys, and the lemurs, the monkeys being divided into the catarrhinians, platyrrhinians, and arctopithecans; and the catarrhinians subdivided into the anthropomorphous and the cynomorphous apes.
Vogt, in his "Mammalia"—first group, man, which we mention here, but which is not treated of; second group, the monkeys of the old continent, divided into anthropomorphous or tailless monkeys, and monkeys with tails; third group, the monkeys of the new continent, divided into platyrrhinians and arctopithecoids; and fourth group, the lemurs or prosimians.
From this we see that, with the exception of Broca, all these authors agree in uniting the great apes or anthropoids under the term apes, or catarrhinian apes, or apes of the old continent; and that Huxley and Vogt agree with Cuvier, Broca, too, may not be so isolated as I have represented him. We should recollect that he never formulated his division as above, but that it is the incontestable result of his teachings, and especially of those of his later years.
I have been led by my own studies, and resting on the differences that appear between man and the monkeys, great and small, drawn from the volume of the brain, the cranial characteristics which are the consequences of it, the facial traits that accompany it, and the characters of the skeleton which are developed in a parallel way—that is, from all the characteristics which I have especially studied—to abandon the classification of Linnæus and take up the one so much decried of Cuvier, against which no serious reproach has been brought except that of the use of the word quadrumanous and the narrow definition of the hand on which its rests. Cuvier may not have been much of a philosopher, but he was first among observers.
When Broca contested the application of the denomination quadrumanous to the monkeys to distinguish them from man, bimanous, he rested on the fact that the presence or absence of the thumb is not enough to authorize the names of hand and foot; that in man, every superior member concurs in the function of prehension, of which the extremity of the member is the immediate organ, while in the inferior member everything is organized with a view to the functions of locomotion and support which the extremity only seems destined to fulfill; in short, that there is a solidarity between all the parts of either limb, the various details of which constitute the characteristics of the functions of hand and foot. This is admirably true, as to man, at the summit of the evolutionary series of which he is the crowning. It ought to be true, too, when we descend the course of the series.
The fore-limbs of the monkeys are indeed adapted to the function of prehension, but they are at the same time organs of locomotion; the hinder limbs are also adapted to walking, but they are at the same time organs of prehension. With the lemurs there are also the same general types of all the limbs, for prehension in front, for walking behind, but the fore extremity is in fact more a paw, and the hinder one more a hand by comparison; witness, for example, the Cheiromys. Other monkeys are as quadrupedal as they are quadrumanous. Consider the three upper segments of each limb: there are indeed an arm in front and a leg behind; but look only at the last segment, and it will be found to be, in front as well as behind, a hand in its principal characteristics, the separated and opposable thumb, and the nails.
With man, the harmony is perfect, because the functions are specialized, and the organs are all adapted in the same respective directions, the fore ones for prehension, the hinder ones for walking. Beyond our branch of the primates, looking toward its origin,, the four limbs all exhibit themselves with the same types, but less affirmed, less precise: all four for prehension, the fore ones more so; all four for walking, the hinder ones more so. The evolution begins after the train of the marsupials, and specializations are made in different directions. With some, as the galeopitheci and the, the particular adaptation took the direction of flight; a part or all of the limb was not transformed, but bent itself to what was required, was obedient to solicitations. With others, as the ungulates, the adaptation took the direction of an exclusive locomotion upon all four limbs. These became gradually modeled upon the same type, the useless bones disappeared or were fused, and some superfluous motions ceased, while others became accentuated, and the necessary corresponding anatomical dispositions with them. With others, as the carnivora, which were to run on the ground to reach their prey, while they must be able at the same time to seize, hold, and tear it, the four paws remained perfect locomotor organs, but at the same time also organs of attack by their claws, and to a certain measure, particularly in the fore extremities, organs of prehension. An adaptation of another kind was produced with the monkeys. The animals from which they sprung dwelt in trees and ran along the branches. They needed to increase their power of prehension, they had to clasp the rounded trunks of trees, to hook on to branches in passing from one to another. The adaptation seems to have appeared first in the hinder limbs, and then in the fore-limbs. The whole of the limb did not have to lose its peculiar type for that; but it was enough if the extremities were in some way fitted to it. Nature was contented with nails, separated and opposable thumbs, and more flexible fingers, without going up to the next segment.
One fine day a revolution was effected. Just as an adaptation to arboreal life was produced at the expense of anterior species, an adaptation to terrestrial life was made, with a bipedal attitude favorable to a more extended vision, a diminution of the olfactory sense and of the facial prominence, a more perfect touch, and intelligence. Henceforth, all the living forces of adaptation tended toward the same end; the hind-thumb ceased to be opposable, the other toes diminished in length; what the feet lost the hands gained, and man was created, exclusively bimanous in front, exclusively bipedal behind, and all the accessory parts in the segments of the limbs confirming themselves in the types, less accented till now, which they had presented since the marsupials.
The peculiarity set forth by Cuvier of the opposable thumb perfectly characterizes what there is common and special among all the apes, the faculty of clinging to trees with the four extremities. It is true that this expresses only one of the details of that whole, perfect in man, which has given birth to the words hand and foot, but it is the essential one. It can not, however, be denied that the second characteristic necessary to the function of prehension—great mobility in every direction of the segments of the limb—is not very greatly developed in the hind-limbs of monkeys. Cuvier had, then, a perfect right to call all the monkeys quadrumana, although they were at the same time quadrupedal, and to oppose them to man. I, then, put the anthropoids and ordinary monkeys together under the name of monkeys, and will not recoil from the synonym of quadrumana if the term monkey does not suffice me.
The monkeys are divided into two groups, those of the old continent, also called catarrhinians, because their nostril-partitions are narrow and their nostrils are open below the nose (from κατα, low, and ῤιν, nose); and those of the new continent, also called platyrrhinians, because their nostril-partitions are broad, and their nostrils open on the side (from πλατνς, flat). The monkeys of the new continent are predominantly tree-dwellers, and are divided into two families—the monkeys proper of this continent, and the arctopitheci. The former are in turn divided into the diurnal—the howlers, the ateli, the sajous, etc.; and the nocturnal, including the sagoins, sakis, nyctipitheci, and the saimiris.
The arctopitheci or hapales are a group apart among monkeys, including the interesting wistit and the tamarin. They are tree-dwellers like the former group, and nocturnal like the latter. They afford an example of the imperfection of our modes of classification. They are monkeys, American monkeys, in many of their relations; but they lack the single characteristic that distinguishes all the monkeys, including the lemurs, and have the dentition neither of the American monkeys nor of the monkeys of the Old Continent. We have removed the galeopitheci from the lemurs on account of the absence of the first character. Must we also remove the arctopitheci from the monkeys? Let us look at their characteristics. When we take hold of their skull in such a way as to hide the lower part of the face, they look exactly like American monkeys. Like the American monkeys, they have a round head, flat face, lateral nostrils, no gluteal callosities, no pouches. But they have not opposable thumbs, either in the fore or hind limbs, and this deprives them of the single characteristic common to all the monkeys and false monkeys. Further, they have claws on all the fingers, except on the hind-thumbs, which alone have nails. They have thirty-two teeth, the same number as the monkeys of the Old Continent and man, but with a different formula—one little molar more and one large molar less. Further, their teeth have some insectivorous characters; the lower canine is small, the molars work a little into one another like those of insectivora, and some, the forward ones, have sharp, conical points. The lower incisors of some species are pointed. Cuvier hesitated to put them among the quadrumana. For our own part, we readily see in them a step toward the primates, a kind of American lemur, a transition from the insectivora to the monkeys of the New Continent.
Fossil monkeys have been found in America, and it is remarkable that they all have thirty-six teeth, and relate themselves to the types of that continent as if the platyrrhinians had always lived there. The highest among them is the Laopithecus, which can be compared to the anthropoids of the eastern continent. In short, we are introduced in America to a special series, constituted, from its origin to its end, thus: Some insectivora; arctopitheci; nocturnal monkeys, beginning with the saimiris; diurnal monkeys; Laopithecus. MM. Vogt, Schmidt, and Cope accept this insectivorous origin.
The monkeys of the old continent are less tree-dwelling than those of the new continent, and are all diurnal. Most of them have pouches and gluteal callosities. Their teeth are generally less omnivorous than those of man, and tend, especially by the canines, to the carnivorous type, and are also less continuous. They are divided into the great monkeys, tailless monkeys or anthropoids, and tailed monkeys, which are again divided into semnopitheci, cercopitheci, and cynocephaluses. The semnopitheci (from σεμνός, venerable) include the entellus, the sacred monkey of India, a prominent figure in the Aryan legends, and the colobus of Abyssinia and Guinea. The cercopitheci include the guenon, which is found only in Africa; the magot, which lives in Africa and as far north as the rock of Gibraltar; and the macacus, which occurs in India and Japan. The cynocephaluses are large monkeys with a dog's snout, of which numerous species inhabit the most of Africa.
The Old World monkeys are related on one side to the lemurs, and on the other side to the ungulates. The former relationship is clearly admitted by Prof. Haeckel and Mr. Cope. M. Haeckel's argument, which is based chiefly on the conformation of the placenta, does not carry a strong conviction. Mr. Cope's rests chiefly on the conformation of the teeth, and is more solid. Mr. Huxley does not say that the monkeys are descended from the lemurs, but his descriptions suggest it. M. Vogt, as we have seen, rejects this genealogy, as also does M. Schmidt. The relationship with the ungulates is admitted by M. Gaudry, and is a consequence of the one that he has determined between the lemurs and the ungulates. In general, the Adapis and the Aplelotherium establish the communication on the former side, the point of junction being at the Eocene origin of the perissodactylic branch of the ungulates. On the latter side we have only one genus still known, the Oreopithecus of Gervais, which in dentition resembles the Chæropotamus, a genus of the Suidæ, or the artiodactylic branch of the ungulates. In return, there are genera of the ungulates belonging to the same stock of the Suidæ, or one nearly allied to it, which have marked resemblances with the monkeys. These are the Cebochærus, or hog-monkey, of Gervais, the Acotherulum, and the Hyracotherium of Owen. It is also to be remarked that in his general demonstration of the relation of the preceding species with the ungulates, M. Gaudry does not separate the lemurs from the monkeys, as if, from the paleontological point of view—that is, in the ancient species—the two were confounded.
Assuredly this is a very slight basis on which to found a derivation of the monkeys, and ultimately of man, from the ungulates. Yet the hypothesis has been heard; M. Vogt seems disposed to accept it, and M. Schmidt concludes a chapter in his book with the words: "The monkeys have had a very distinct double origin; the American branch had ancestors of insectivorous forms, and the Europo-Asiatic branch, including the anthropomorphs, ancestors with pachydermatous forms. We are thus near the question of the pachydermatic origin of our own primitive ancestors."
If this be so, the catarrhinian monkeys are dispossessed of their filiation with the lemurs. I confess I can not make up my mind to accept this idea. The lemurs are to me primates, quadrumana, the lowest of the order, and as such the ones which have all the chances of having engendered the others. The theory of the descent of man from the hog does not seduce me.
I am an anatomist and craniologist, and will allow no one to cast doubt on the importance which I attach to the smallest morphological feature; but I ask, if over and above the details of the conformation of the teeth, fingers, and toes, the tarsus and carpus, above the characters that reflect the exact kind of alimentation, the precise method of locomotion, there is not something more general, answering to special habits, to more or less aërian, terrestrial, aquatic, diurnal, or nocturnal ways of life or abode, which impresses on the totality of the organism that general family resemblance which the naturalist recognizes outside of all those special modes of adaptation, which he studies with so much care to find in it a testimony, an expression, a formula in support of his thought and vision. A particular trait, a progressive variation of form, it is evident, reflects in general the elevated influence to which I allude. The teeth, the condyle of the jaw and its articular cavity, the temporal fosses, express quite exactly the regimen of the animal, and consequently some of its habits. The patagium, of which some traces have been observed among the petaurite marsupials, permits us to establish a series leading to the bats and passing by the galeopitheci. The genealogy of the perissodactyli, one of the most satisfactory that science has determined, rests essentially on a single character, the number and degree of atrophy of the fingers or toes.
But is the chosen form of character all? Has not nature different ways of reaching the same end, and can it not distribute its influence over the whole of the organism without making any of the characteristics particularly distinctive, and even while leaving present seemingly contradictory ones? The mouse is recognized everywhere by its attitude, its walk, its head, and its general shape, and still is found under different names among the aplacental and the placental orders, with the rodents and with the insectivora, terrestrial, half-aquatic, half-flying, and flying. The same is the case with the genus squirrel, which is scattered, with changed names, among several orders, being simply modified in some peculiarity. There is a group of most remarkable leaping animals among the marsupials, which, while preserving its type, is distributed, according as it acquires certain new characters, among various placental orders.
I ask, then, if the peculiar bearings of the monkeys, if their habitat, exclusively in trees among their most pronounced representatives, which impresses a special stamp on the whole individual; if the proportions of their body, the extent and situation of the articular surfaces and the consequent mobility of the segments upon one another, do not furnish a sufficient motive for establishing their relationship with the lemurs, and not with the ungulates? Likewise the lemurs, which lead a similar life, conduct to the marsupials, which also constantly inhabit trees. Between the ungulates and the monkeys I see nothing common of the same kind. I can not imagine an animal with hoofs, walking only on the ends of its toes, having metatarsi joined, lengthened, and raised up, with the four limbs brought close to the body, and moving nearly always in the same parallel plane—that is, adapted to a terrestrial, measured, and rhythmical locomotion—giving birth to an animal with nails, plantigrade, having movable fingers made to fit themselves around trees, to hook on to branches, with limbs endowed with the most unrestricted movements of abduction and adduction. But it requires no mental effort to conceive an adaptation already begun in this direction with the lemurs, and having only to continue and specialize itself still further in the monkeys.
- From a lecture at the École d'Anthropologle, March 21, 1888.