The thumb is not seen in the least in one kind of Colobos, the true Colobos (Colobos verus); in others it is like a little knob, but in none is it of any use. In the corresponding member of other Monkeys there are three bones, one placed before the other. The first, the metacarpal, is the nearest the wrist, and is jointed to the wrist-bone called trapezium, and in front it is in contact with the second bone, or the first phalanx of the thumb. This is ended by the second phalanx, which bears the nail. These are terms used by anatomists, and the word metacarpal means “the next in order of rank to the wrist.” These metacarpal bones intervene between the knuckles and the wrist, and are long and parallel with each other, there being five in the hand. They are not usually very movable on the wrist, but that of the thumb is, and they have a joint at the further end which unites them with the so-called internode or phalanx-bone, No. 1. The word internode means between joints, and the term phalanx is one of those unmeaning applications of Greek terms which abound in anatomy. The phalanx was an order of battle, and means rows placed in parallel order: the internodes of the fingers, when in place, are one before the other and side by side, like the soldiers in the Greek order of battle. Each phalanx represents a bone: there are two in the thumb, and three in the other fingers. In the Colobos there is a joint on the wrist-bone for a thumb, but no thumb exists, but there is just a little vestige of a bone, and it is probably the first phalanx, or internode, and not the metacarpal.
The thumb is therefore “rudimentary” in the genus Colobos, and why? The animals are tree-climbers and active jumpers, and can run very well on all-fours; in fact, their method of life and of motion is that of the Monkeys which have well-formed thumbs. The notion of a useless organ is at first repulsive to our ideas of the benevolent scheme of Nature. Mr. Darwin writes, “In reflecting on them every one must be struck with astonishment; for the same reasoning power which tells us plainly that most parts and organs are exquisitely adapted for certain purposes tells us with equal plainness that these rudimentary or atrophied organs are imperfect or useless.” Let us take a well-known instance of such a structure: the Calf when born has cutting teeth in its upper jaw hidden in the gum; they are not in sockets, and even if they were, they would be of no use in biting. The Ox has no cutting or incisor teeth in its upper jaw, as every one knows, and the tongue touches a hard and moist gum there. The incisor teeth of the Calf are never cut, but they are gradually absorbed in the gum with age. Now what is their meaning? They are of no use in sucking, or in anything which occurs in the early life of the animal: they are clearly useless and rudimentary or atrophied structures. Take another example: the little Kiwi bird of New Zealand has no wings with which to fly, yet the bones are there in a dwarfed and rudimentary condition; many insects have no wings, or have them so reduced in size that they are of no use in flight, and sometimes the males have them in perfection, and the females have none. In explaining this subject two courses are open, first, to beg the question, and to say that the design of the Creator was thus; or to account for it on the principle that the Creator acts by law, and that creatures become modified and altered by inherent power, and by having to obey the force of surrounding circumstances generation after generation.
COLOBOS VERUS. (After Van Beneden.)
In the instance of the male and female insect just noticed, the male is active, and has to search for his partner, and the female is a stay-at-home, and expects to be courted, and when mated to do nothing more than lay eggs. Her wings would be of doubtful value. We may believe, then, that disuse, generation after generation, gradually weakened the wing, and finally Nature, ever economical in not-used organs, did not perpetuate it. Disuse may be therefore considered as the principal cause of the atrophy, rudimentary condition, and of the final deficiency of structures. But disuse will not produce this in one generation, but in many, so it is necessary to look farther back into the ancestry of the creatures which have rudimentary organs. The four-legged ruminating or cud-chewing animals have bones and feet of peculiar arrangement, and there is no difficulty in at once knowing a ruminant by its bones. Now, in former ages, and before there was a trace of man on the globe, there were ruminants, as known by their bones found in strata or deposits, and they had incisor teeth in their upper jaws when full grown, and not only when in the calf condition. The inference to be drawn is, that the modern Oxen are the descendants of those ancient forms with incisor teeth, and that disuse, probably produced by the introduction of grass-feeding on a grand scale, instead of leaf-and bud-nibbling, gradually diminished the strength and permanence of the front upper teeth, and finally only left the simple traces of them which we have mentioned. Disuse by ancestral forms, by the forefathers, and the carrying down the weakened and atrophied state of the structure or organs, are the most important considerations in any attempt at the explanation of the seeming paradox. In endeavouring to apply this style of reasoning to the Colobos group—the Semnopitheci without thumbs—it must be asked, is there any evidence of the great antiquity of these Monkeys, and are there any evidences of anything wrong about the thumbs of their Asiatic allies?
GUEREZA.
It is remarkable, and bears strongly upon this point, that some of the fossil remains of animals found in India, on the flanks of the Himalayan Mountains, have a closer resemblance to a large Semnopithecus Monkey than to any other, and to one belonging to a kind much like the Entellus. The bony remains were found in collections of shingle, clay, and sand of great depth, and which included also the remains of the bones of Elephants, Giraffes, Hippopotamidæ, Crocodiles, and fresh-water Tortoises, and other land and fresh-water creatures. The deposits had accumulated in lakes and swamps in the plain near the distant flanks of a low range of hills, the ancient foundations of the present great snowy range, and then upheaval took place, which gave the very home of snow (Himalaya) its present vast altitude. The plains, lakes, and swamps were lifted up and tilted, and their relics are now found resting at a considerable angle on the main chain, and covered and folded over by the pressure exercised during the marvellous change in the physical geography of the district. Semnopitheci lived in India, then, before the Himalayas were a great chain of mountains, and they lived with animals which were African as well as Asiatic in their character. The vast age of the groups of Monkeys must be admitted, for the Himalayas are as old as the Alps, and as both have been worn down into their present condition of peak, pass, and valley since they were uplifted, their age is incalculable by years. The former connection of Africa and Asia by means of intermediate land, which is now the floor of the Indian Ocean, to the west of Hindostan, may be reasonably asserted to have been severed at the same time when the mountains far away to the north-east received their breadth and height. So that before these great terrestrial changes occurred, Semnopitheci could have either an Indian or an African home. Disuse of the fore-thumbs in branch-crawling or swinging may then have commenced before that geological age in which these things happened, and it may have progressed very decidedly in Africa, and not so much in Asia. Hence the Semnopitheci here have rather small thumbs, and the African groups, separated by the physico-geographical change, and disusing generation after generation, have gradually lost the structure.
The Colobi resemble the Semnopitheci in the construction of their compound-looking stomach.