Popular Science Monthly/Volume 33/October 1888/Curiosities of Evolution
|CURIOSITIES OF EVOLUTION.|
NOTHING is more strange in the history of evolution than the persistence of rudimentary structures, which have lost all usefulness untold generations ago, and in many cases have become absolutely dangerous to the organism. Among these survivals, one of the most curious is the pisiform bone of the wrist, which careful researches in comparative anatomy show to be the carpal or wrist bone belonging to a long-vanished sixth finger. The oldest mammals discovered have never more than five fingers. It is necessary to go back to amphibian forms to find a sixth finger, yet all mammals possess the wrist-bone formerly belonging to it. The pineal gland, once supposed, for want of a better hypothesis, to be the seat of the soul, is a still more curious instance of survival, inherited probably from some transparent invertebrate ancestor with a median eye.
In mammals the pineal gland is deeply sunk beneath the highly developed intellectual portion of the brain, in a position utterly cut off from all possible communication with the outer world. Human physiology alone would have left us utterly without a clew as to the original use of this mysterious body. The secret was discovered after long and patient study of the brains of amphibians and reptiles. In these animals the intellectual portion of the brain (cerebrum) is in a very undeveloped condition, and the pineal gland is not covered. It lies just beneath the parietal suture, that portion of the top of the skull where the bones are still ununited in new-born children. In many reptiles, notably Hatteria (Fig. 1) and Aurelia, the pineal gland is found to be an optic lobe, united to the nerve-stalk of a true eye, richly supplied with a branched blood-vessel and nerve. Although this eye still possesses every essential part of a visual organ, yet degenerative changes have set in, which show that it has been long useless. In Varanus giganteus (Fig. 2), where a scale on the top of the head is fitted by its transparency and whiteness to act as a cornea, a large mass of pigment has accumulated just beneath, effectually preventing the possibility of any rays of light reaching the retina. In Hatteria, the eye appears fitted in all respects for vision, but a thick band of connective tissue has formed above it, and there is no modified scale. In both animals, Hatteria and Varanus, the rods and cones of the retina are strangely elongated in certain parts, as though from straining to catch the last rays of vanishing light. The rods of this portion are at least three times the length of the ordinary ones, and are in connection with a special group of nucleated cells.
In modern amphibians the greatly degenerated eye is separated entirely from the pineal stalk, though a connection still exists during embryonic life. But there is reason to think that among ancient amphibians—more especially among the labyrinthodonts—the pineal eye reached its very highest development, since it is found outside the skull. A large parietal opening, with roughnesses of the skull-bones serving as attachments for powerful muscles, is found in the great extinct amphibians and reptiles. The pineal eye was pre-eminently a sense-organ of pre-tertiary periods; it has probably never been functional since these remote ages, and yet its rudiments persist in every human brain. Moreover, these eyes are of the invertebrate type, pointing back to that conjectural molluscoid ancestor which was "transparent and had a median eye."
The records of pathology teem with instances of rudimentary organs which have lost their use and have become sources of danger and disease. I venture to think these facts are far too little known to those outside of the medical profession who are interested in evolution. It is not necessary to do more than allude to the "appendix vermiformis," since every reader of the "Descent of Man" will remember it as the typical instance of a mischievous rudimentary organ, given by Darwin.
All mammals possess, during their embryonic life, three sets of kidneys. The first set of tubules cease very early in fetal life to act as kidneys; they take on a new function of supreme importance, their ducts becoming the oviducts in most fishes, amphibians, reptiles, and birds, and constituting the uterus and Fallopian tubes in mammals. In their earliest stage they are known as the pronephros, or "head-kidney" and answer to the permanent condition of the renal organs in worms; in their second stage they are known as the "ducts of Müller."
The second set of tubules constitute the mesonephros or Wolffian bodies; they act for a time as kidneys, and then become
the ducts of the generative organs in the male. In the female they have no later functions, but their atrophied remains persist, and give rise to various forms of cystic disease. The upper division of the Wolffian duct, with its tubes, can be found lying above the ovaries, and is known as the parovarium. It is frequently the seat of degenerative disease, not only in human beings, but in lionesses, tigresses, and cows. The middle portion often disappears, but in the cow the whole tube persists, useless always, and mischievous very frequently. Both sets of tubules, those of the pronephros and those of the mesonephros, or, in other words, the ducts of Müller and the Wolffian ducts, persist throughout life in both males and females, one set becoming highly developed, and the other atrophying, according to sex. This fact, of course, points back conclusively to primitive ancestors of fishes, reptiles, birds, and mammals, which were hermaphrodite. The argument for design is utterly put out of court by the awkwardness of the whole plan, and the disease and suffering caused in mammalian females by the ovaries having no
original connection with the uterus and by the survival of useless rudimentary organs. As a blind effort of Nature in the process of converting a hermaphrodite worm into a warm blooded mammal, the process has its wonderful and admirable side. The third set of kidneys are, of course, the permanent ones.
Prof. Cope, in his "Origin of the Fittest," draws attention to what he names the "law of acceleration and retardation." This law, though it may indirectly lead to the "survival of the fittest," is equally likely, through its blind action, to lead to the extinction of an animal which had once been the "fittest" in its relation to the environment. There can be little doubt that the enormous tusks of the early elephants and the formidable canines of many early carnivores would enable them at first to distance all competitors. But the law of acceleration tended blindly always in the same direction, till the old elephants seem to have been weighed down by their extravagant tusks, and the most highly specialized of all carnivores had canines so long that they could not shut their mouths, and both speedily became extinct. The law of retardation exhibits itself in the teeth of the higher races of mankind in a highly inconvenient manner. The greatly developed brain requires all the available room in the skull; there is no space left for the attachment of muscles for a powerful jaw. Cooked food also causes a degeneracy in the development of the jaw. There is constantly no room left for either the wisdom-teeth or the second upper incisors; the wisdom-teeth are retarded, often cause great pain, and decay early. The second incisors appear in startling and unexpected places, and often (in America especially) do not cut the gum at all. Prof. Cope says that "American dentists have observed that the third molar teeth (wisdom-teeth) are in natives of the United States very liable to imperfect growth or suppression, and to a degree entirely unknown among savage or even many civilized races." The same suppression has been observed in the outer pair of superior incisors. This is owing not only to a reduction in the size of the arches of the jaws, but to successively prolonged delay in the appearance of the teeth. In the same way men, and the man-like apes, have fewer teeth than the lower monkeys, and these again fewer than the insectivorous mammals to which they are most nearly allied. When this difference in dentition has been established, civilized man may claim to place himself in a new species, apart from low savages as well as from the high apes.