Descent (biology)

“ Do the races or species of men, whichever term may be applied, encroach on and replace one another, so that some finally become extinct? We shall see that all these questions, as indeed is obvious in respect to most of them, must be answered in the affirmative, in the same manner as with the lower animals. But the several considerations just referred to may be conveniently deferred for a time: and we will first see how far the bodily structure of man shows traces, more or less plain, of his descent from some lower form. ”

“ This much by way of preliminaries being understood, we have next to notice that whichever of the two rival theories we choose to entertain, we are not here concerned with any question touching the origin of life. We are concerned only with the origin of particular forms of life—that is to say, with the origin of species. The theory of descent starts from life as a datum already granted. ”

“ The chief evidences of the theory of descent are to be found in homologies, in the correspondences of organs and functions, as revealed by comparative anatomy and physiology, in the recapitulation revealed by embryology, in the structure of parasites, in rudimentary organs and reversions to earlier stages, in the distribution of animals and plants, and in the possibility of still transforming, at least to a slight extent, one species into another, by experimental breeding.
Transformation and differentiation go on in nature as a vast, ceaseless, but blind process of selection. ”

“ In each case of what we call descent, it is still the first reproducing creature identically the same—doing what it has done before—only with such modifications as the struggle for existence and natural selection have induced. ”

“ We can generally form a clear conception why this should be the case; for not only is there in another species a different kind of latent vital activity, but each species has also a different developmental history. ”

“ In trying to establish any special hypothesis of descent, whether by the graphic method of drawing up genealogical trees or otherwise, let us always bear in mind Darwin's words ("Descent of Man", page 229.) and use them as a critical guiding line: "As we have no record of the lines of descent, the pedigree can be discovered only by observing the degrees of resemblance between the beings which are to be classed." ”

“ As to what the Darwinians think, let me quote from my book, "Darwinism and the Doctrine of Descent": "The grade to which this (intellectual) development rises is generally dependent on the preceding generations. The psychical capacities of each individual bear the family type, and are determined by the laws of heredity. ”

“ In this, and other instances presently to be given, the cause of ancient races approaching the lower animals in certain characters more frequently than do the modern races, appears to be, that the latter stand at a somewhat greater distance in the long line of descent from their early semi-human progenitors. ”

“ In all detailed investigations into an animal type, in the study of the structure, functions, or the instincts of an ant, or of a whale or of a tape-worm, the standpoint of the theory of descent is assumed, and it proves a useful clue for further investigation. ”

“ We have now reached the end of our studies, and they have given us satisfaction, at least in so far that they have brought us certainty in regard to the chief and fundamental question which can be asked in reference to the origin of the modern animate world of organisms. There remains no doubt in our minds that the theory of descent is justified ”

“ Thus we can understand how it has come to pass that man and all other vertebrate animals have been constructed on the same general model, why they pass through the same early stages of development, and why they retain certain rudiments in common. Consequently we ought frankly to admit their community of descent: to take any other view, is to admit that our own structure and that of all the animals around us, is a mere snare laid to entrap our judgment. ”

“ Finally, we have seen that natural selection, which follows from the struggle for existence, and which almost inevitably leads to extinction and divergence of character in the descendants from any one parent-species, explains that great and universal feature in the affinities of all organic beings, namely, their subordination in group under group. We use the element of descent in classing the individuals of both sexes and of all ages under one species, although they may have but few characters in common ”

“ Most persons even now refuse to acknowledge the most important deduction of the Theory of Descent, that is, the palæontological development of man from ape-like, and through them from still lower, mammals, and consider such a transformation of organic form as impossible. But, I ask, are the phenomena of the individual development of man, the fundamental features of which I have here given, in any way less wonderful? ”

“ Although the majority of experts in anatomy and zoology accept unconditionally the descent of man from lower forms, there is much diversity of opinion among them in regard to the special line of descent. ”

“ But where, as in the present section, we treat the descent theory apart from the evolution theory, we have also to think of the possibility that the species or groups of species are not originated through gradual development, but nevertheless do originate through descent—namely, in leaps through metamorphosis of germs or a heterogenetic generation ”

“ A third great step was implied in the acceptance which Darwin in particular won for the general idea of descent, for it is hardly too much to say that the scientific study of the problems of heredity began when it was recognised that heredity is a fundamental condition of evolution. ”

“ The principles of classification point also in the same direction, and are based more and more on the theory of descent. Biologists endeavor to arrange animals on what is called the "natural system." No one now places whales among fish, bats among birds, or shrews with mice, notwithstanding their external similarity; and Darwin maintained that "community of descent was the hidden bond which naturalists had been unconsciously seeking." ”

“ Alfred Russel Wallace, have lately maintained that though the theory of descent with modification accounts for the development of all vegetable life, and of all animals lower than man, yet that man cannot—not at least in respect of the whole of his nature—be held to have descended from any animal lower than himself, inasmuch as none lower than man possesses even the germs of language. ”

“ The theory of descent is the hypothetic statement that the organisms are really allied by blood among each other, in spite of their diversities.143 The question about their so-called monophyletic or polyphyletic origin is of secondary importance compared with the statement of relationship in general. ”

“ If it can be shown possible for man to have descended or ascended from the lower animals, it will require enormous additional evidence to show that such descent is probable; and still much more to make it certain. ”

“ In seeking to define our position in regard to the theory of descent it is most important that we should recognise that, when it is looked into closely, the true problem at issue is not a special zoological one, but is quite general, and also that it is not a new growth which has sprung up suddenly and found us unprepared, but that it is very ancient and has long existed in our midst. ”

“ Descent with modification is now universally accepted as the chief law of nature in the organic world. In honor of him, who with unsurpassed genius, and by unlimited labor has made it the basis of modern thought, this law is called the "Darwinian theory of descent." ”

“ It is true that this country is not entirely unknown, and if I am not mistaken, Charles Darwin, who in our time has been the first to revive the long-dormant theory of descent, has already given a sketch, which may well serve as a basis for the complete map of the domain ”

“ It must rather depend on causes innate in the organism itself, i.e., on inherited duplicating tendencies which meet in the same generation, and to a certain extent contend with each other for precedence. ”

“ The facts, showing that with certain diseases the period of inheritance occasionally or even frequently advances, are important with respect to the general descent-theory, for they render it probable that the same thing would occur with ordinary modifications of structure. ”

“ These naturalists thereby designate a gradual bodily transformation appearing in the course of generations in correspondence with the new requirements of altered conditions of life or, in other words, the action of natural selection, and not the result of a suddenly and direct acting transforming cause exerted but once on a generation. ”

“ Weismann has for many years occupied himself. I may however point out that before the appearance of the present work the author, in addition to his well-known papers on the embryology and development of insects, had published two valuable contributions to the theory of descent, viz. one entitled “Über die Berechtigung der Darwin’schen Theorie” (1868) , and another “Über den Einfluss der Isolirung auf die Artbildung” (1872) . ”

“ Such social qualities, the paramount importance of which to the lower animals is disputed by no one, were no doubt acquired by the progenitors of man in a similar manner, namely, through natural selection, aided by inherited habit. ”

“ The successive stages of transformation of one and the same physiological individual84 do not in these cases serve as the point of departure for alternation of generation, but the different contemporary forms living gregariously into which the species has become divided through functional differentiation of the various individuals of the same stock. ”

“ The antithesis of nature and nurture is not a new one; it was met long ago by biologists and settled by them to their own satisfaction. The whole body of experimental and observational evidence in biology tends to show that the characters which the individual inherits from his ancestors remain remarkably constant in all ordinary conditions to which they may be subjected. ”

“ No doubt, in this case, and in all others, many checks concur, and different checks under different circumstances; periodical dearths, depending on unfavourable seasons, being probably the most important of all. So it will have been with the early progenitors of man.
Natural Selection.—We have now seen that man is variable in body and mind; and that the variations are induced, either directly or indirectly, by the same general causes, and obey the same general laws, as with the lower animals. ”

“ We shall then, if in fact the theory of descent be correct, be able from his position in the system to determine the real primary relationship, and the degree of consanguinity connecting Man with the animals most like him. The hypothetical pedigree of the human race will then follow naturally as the final result of this anatomical and systematic inquiry. ”