Monkeys have always been a subject of idle interest to old and young; but they have usually served to amuse the masses more than to instruct them, until within recent years.

Now that science has brought them within the field of careful research, and made them an object of serious study, it has invested them with a certain dignity in the esteem of mankind, and imparted to them a new aspect among animals.

There is no other creature that so charms and fascinates the beholder as do these little effigies of the human race. The simple and the wise are alike impressed with their human look and manner; children and patriarchs with equal delight watch them with surprise; but now that the search-light of science is being thrown into every nook and crevice of nature, human interest in them is multiplied many fold, while the savants of all civilised lands are struggling with the problem of their possible relationship to man.

Pursuant to the desire of learning as much as possible about their natural habits, faculties, and resources, they are being studied from every available point of view, and every characteristic compared in detail to the corresponding one in man. Hence, in order to appreciate more fully the value of the lessons to be drawn from the contents of this volume, we must know the relative planes in the scale of nature that man and monkeys occupy, wherefore we shall begin our task by comparing them in a general way; but as the scope of this work is restricted mainly to the great apes, the comparison will likewise be confined to that subject, except in so far as to define the relations of man and ape to monkeys.

Since monkeys differ among themselves so widely, it is evident that all of them cannot in the same degree resemble man. And as the degree of interest in them as a subject of comparative study is approximately measured by the degree of their likeness to man, it is apparent that all cannot be regarded as of equal interest. But since each forms an integral part of the scale of nature, they are of equal importance in tracing out the continuity of the order to which they belong.

The vast family of simians has perhaps the widest range of types of any single family of mammals. Beginning with the great apes, which so closely resemble man in size, form and structure, they descend by degrees along the scale till they end in the little marmosets, which are almost on the level of rodents. But the descent is so gradual that it is difficult to draw a sharp line of demarcation at any point between the two extremes. There is, however, now an effort being made to separate this family into smaller groups, but the lines between them must be dim and wavering, and the literature of the past has a tendency to retard the effort.

We shall not digress from the trend of our subject, however, at this time, to discuss the problems with which zoology may have to contend in the future, but will accept the current system and proceed.

All the varied types that belong to the simian family are, in the common order of speech, known as monkeys, but the term thus used is so broad in its meaning as to include all the forms of that vast group, wherefore it is vague and obscure, for some of these resemble man more than they resemble each other. The name should only be applied to those having tails and short faces, but there is a small group, which have no tails at all, that are properly known as apes. While they are all simians, they are not all monkeys. It is with this small group, without tails, that we propose chiefly to deal. We select them because of their likeness to man, and having noted the similitude, the result may be compared with other types of the same order. There are only four of these apes, but as a whole they resemble man in so many essential details that they are called "anthropoid," or "man-like apes." They differ from each other in certain respects, almost as much as any one of them differs from man. The four apes alluded to, are the chimpanzee, the gorilla, the orang and the gibbon.

As the skeleton is the framework of the physical structure, it will serve as the basis upon which to build up the comparison, and as the chimpanzee is the nearest approach to man, we select him as the highest type of the simian, and use him as the standard.

The skeleton of the chimpanzee may be said to be exactly the same as that of man, but the assertion must be qualified by a few facts which are of minor importance, but since they are facts we cannot ignore them.

The general plan, purpose and principle are the same in each. There is no part of the one that is not duplicated in the other, and there is no function discharged by any part of the one that is not discharged by the like part of the other. The chief point in which they differ is in the structure of one bone.

Near the base of the spinal column is a certain bone called the sacrum. It is a constituent part of the column, but in its singular form and structure somewhat differs from the corresponding bone in man. The general outline of this bone in the plane of the hips is that of an isosceles triangle. It fits in between the two large bones that spread out towards the hips, and articulate with the thighbones.

About half-way from the centre to the edge, along each side, is a row of four round holes. Across the surface of the bone is a dim transverse line between each pair of holes, from which it appears that five smaller sections of the column have anchylosed or grown into each other to form the sacrum, and the holes coincide with the open spaces between the lateral processes of the other bones of the column above.

In the chimpanzee, this bone has the same general form as in man, but instead of four holes in each row it has five, connected by transverse lines in the same way, indicating that six of the segments are united instead of five; but to compensate for this the ape has one vertebra less in the section of the column just above it, in that portion called the lumbar. In it man has five, while the ape has but four. But counting the whole number of bones in the spinal column, and regarding each segment of the sacrum as a distinct bone, which to all intents it is, the sum of the bones in each column is exactly the same.

Although this appears to be a fixed and constant character, it cannot be esteemed as a matter of great importance, since the same thing has been known to occur in the human skeleton, and the reverse has been known in some specimens of the apes, but has never been observed in the chimpanzee. In this respect he appears to be more constant than man so far as we know at present.

As the greatest strains of the spinal column are laid upon that part in which the sacrum is located, there is a tendency for these segments to unite in order to meet the demand, and since there is the least flexure in that part, the cartilages that lie between them ossify and become rigid. The erect posture of man allows more room in the loins for the fifth vertebra to move, and thus it is prevented from uniting with the segment below it, which is held firmly in place by the two large bones mentioned, while the crouching habit of the ape presses that vertebra firmly against the other, confining it between the two large bones and thus reducing its movement, wherefore the same result follows as with the other sections below.

Another bone that may be said to differ in structure is that known as the sternum or breastbone; it is the thin, soft bone to which the ribs are joined in the front of the body. In the young of both man and ape it is a mere cartilage which slowly ossifies from the top downward. The process appears to begin at different centres, the largest nucleus being at the top. There appear to be five of these centres. The bone never becomes quite hard in either man or ape, but always remains somewhat porous, and even in advanced age the outline of the lower part is not defined by a smooth, sharp line, but is irregular in contour and merges or blends into the cartilages that hold the ribs in place.

In man, this bone in maturity is usually found in two segments, while in the ape it varies. In some specimens it is the same as in man, while in others it is found to be in four or five segments. But the sternum in each is always regarded as one bone, and is developed from one continuous cartilage. The separate parts are never considered distinct bones. The reason that it is found in separate sections in the ape is doubtless due to the stooping habit of the animal, by which the bone is constantly flexed and alternately straightened. In man this bone varies to a great extent.

With these trifling exceptions in point of structures alone, the skeletons of man and ape may be truly said to be exact counterparts of each other, having the same number of bones, of the same general type arranged in the same order and articulated in the same manner. The corresponding bone in each is the same in design and purpose. The frame of the ape is much more massive in its proportions than that of man, but while this is true of some kinds of ape the reverse is true of others. The average height of the adult chimpanzee is about 63 inches.

In man the sacrum is more curved in the plane of the hips than it is in the ape, while the bones of the digits in man are straighter. The arms of man are shorter than the legs, while in the ape these features are reversed.

In the cranial types, it is readily seen that the skull of man is nearly round and the face is vertical, while the skull of the ape is elongated and the face receding. These facts deserve more notice than the mere mention of their being so.

In the whole scheme of nature certain laws obtain in the projection of skulls. The angle between the plane of the face and the spinal axis is co-ordinate to the angle between the spinal axis and the perpendicular.

To be more exact, the spine of a snake is in a horizontal line, and the face occupies a plane of the same kind. At the other end of the scale is man, whose spine is in a vertical line, and his face occupies a like plane. Between these two extremes are types which tend in various degrees, from the lower to the higher form, and just in proportion as the spinal axis approaches a vertical line from one side, the plane of the face approaches it from the other.

In accord with this fact it will be observed that the foramen or hole in the base of the skull through which the spinal cord passes is adjusted closer and closer to the centre of the base of the skull as the spine becomes erect. In man, whose spinal column is erect, the hole is in the centre of the base; in the reptile, whose spine is horizontal, the hole is at the extreme end of the base. In the ape the spinal axis is at an angle with the vertical line, and the plane of the face conforms to a similar one. In keeping with this law it will be seen in all animals that just in the same degree as the angles widen, the foramen is removed from the centre of the base towards the occiput.

It may be noted here, however, that the facial angle is never exactly the same as the spinal angle. The facial plane of the reptile is not quite horizontal, nor that of man quite vertical, but the ratios of angularity are constant. Even the habit of rearing modifies to some extent this character, but it is only the normal pose of the animal that determines the exact limit of it.

In keeping with these facts it will be observed that as the angle between the chin and the spine widens, the lower jaws project, and the chin recedes or flattens, and in a like degree the voice is modified. The chin of man forms a right angle, but in the reptile it is quite lost. In the former the vocal powers are superior to that of all other animals, but as we descend the scale they are reduced in scope and degraded in quality, until in the lowest reptiles they become a mere hiss or squeak. By a careful study of the voices together with the skulls of animals, it is found that the gnathic index can be relied upon as a vocal index. The ape has the smallest angle between the spinal axis and the facial plane, and has the greatest vocal range and purest voice of any other animal below man. Among the apes the gibbon has the smallest angle, and he also has the best vocal qualities of any other ape.

The contour of the skull in all parts conforms to the angle of its projection from the spinal axis. It is depressed and elongated in proportion as the angle increases: the brain cavity is narrowed in a like proportion to its length, and the brain, of course, is modified in the same manner.

The brain of the ape resembles that organ in man as closely as his skeleton resembles man's. It has the same lobes, convolutions, and centres. The texture is slightly coarser. The small details are less intricate and their lines somewhat less distinct. But these also differ to a certain extent in different men. In man and apes the same nerves are present and connect the same organs of sensation, volition and motion. In all essential points they are one.

These leading facts are deemed sufficient to show the physical likeness of apes to man, and we shall refrain from the minute details that would only be of interest to the specialist. The purpose is to acquaint the general reader with the leading facts.

Regarding man purely in the light of an animal, it is evident that he is, physically, very closely allied to the chimpanzee, and that both are integral parts of one great scheme of life, designed by the same author, fashioned after the same model, projected upon the same plan, and amenable to the same system of vital economy. Viewing him in the light of his physical nature, so far it is found that he does not materially differ from other animals in the structure of his skeleton and certain concomitants.

In the vital organs of the two there is perhaps still greater unity of structure, and equal unity of function in all essential details. The difference of structure is only to the extent of making the organ conform to the general plan of the animal, and the difference of function is only one of degree. Since the same characters vary quite as much among men without changing their identity as such, it cannot be sufficient ground to widen the hiatus between man and ape; in fact, the physical likeness of the two grows stronger as the comparison is extended into more minute and scrutinising details. To the casual observer the general resemblance is apparent, but to the student the unity becomes evident.

In addition to the facts we have cited, the ape has the same habits of rest and sleep; lives on the same kind of diet, which is eaten and assimilated in the same manner as with man; is subject to many of the same diseases which attack the same organs, and affect them in the same way as with man; he suffers like pains and dies in the same manner as man under like conditions.

The scope of this book is intended only to embrace the chimpanzee and gorilla, but the comparison which we have shown applies in the name to all four of the anthropoid apes, but must be qualified in a few instances to make it apply to the others. These apes differ among themselves in certain respects in form and habits, and we will omit a detailed comparison of the monkeys as not being relevant to the subject in hand; but it will not be out of place to mention in a general way the chief point in which they differ from men and apes.

There is no fixed type that will represent all kinds of monkeys.

Within the limits of their own family they present a great variety of types, but the one marked difference between them as a unit, and the ape as another, is, that the spinal column of the monkey is always extended into a tail, the first vertebra of which is joined to the base of the sacrum, while the ape has no tail, but the spinal column terminates with a small pointed bone called the coccyx, exactly the same as in man. The number of bones and the number of ribs in monkeys differ from those in the ape or in man, and also vary among different types of monkey.

There are many little shades and grades of difference all along the line, but the unity of design throughout the whole range of simian life is such as to show a continuity of plan and purpose in all essential details of the animal economy. With man and ape the physical structures are one, so far as they pertain to autonomy: their habits are one, so far as they pertain to the means of life; their faculties are one, so far as they pertain to the animal polity, yet they may not be of a common stock.

The public mind does not seem to have grasped the correct idea of evolution, and prejudice has blinded, to some extent, the judgment. The common opinion that man has descended from or is related by consanguinity to a monkey is silly and absurd. Science has never taught such folly, nor advanced any theory from which such a conclusion could be justly deduced. It would be a waste of time for me to offer to explain the doctrine of evolution to any one who does not already understand it from the literature of others on this subject. If he still nurse the idol of the identity of man and monkey, he must be too obtuse or too perverse to be reclaimed. But no one will deny the physical resemblance between man and the great apes, and it is this resemblance we seek to show rather than trace any relationship based upon theories. It is not a matter that concerns the purpose of this work, and we shall here dismiss the subject by saying, that things may be equivalent and yet not identical.