Reproduction

“ Reproduction.—The most fundamental one of the activities for the maintenance of the race is reproduction. Every living organism, whether plant or animal, possesses the power to reproduce its kind. Some plants produce spores and some produce seeds. ”

“ Origin of sexual reproduction; Advantage of sex in chance of survival; Germ and body cells; Limitations of biology in social problems; Sex always present in higher animals; Sex in mammals; The sex problem in the human species; Application of laboratory method.
Sex, like all complicated phenomena, defies being crowded into a simple definition. In an animal or plant individual it is expressed by and linked with the ability to produce egg- or sperm-cells (ova or spermatozoa) . Sexual reproduction is simply the chain of events following the union of the egg and sperm to produce a new individual. ”

“ The term Reproduction has, no doubt, been applied, by different writers, to both these functions;—to the processes by which an organ when mutilated, is restored by the forces of the living body, and to the process by which a new generation of individuals is produced which may be considered as taking the place of the old generation, as these are gradually removed by death. ”

“ Having once originated, the sexual type of reproduction possessed a definite survival value which assured its continuation. Sex makes possible a crossing of strains, which evidently possesses some great advantage, since the few simple forms which have no such division of reproductive functions have undergone no great development and all the higher, more complicated animals are sexual. ”

“ The fact that in all animals but the human species the act can be performed only when reproduction is possible, proves that in the animal kingdom in general the sole object of the function is reproduction. Whether man is an exception, must be determined from other considerations.
2. The fact that the males of other animals besides man in which the sexual organs are in a state of constant development do not exercise those organs except for the purpose of reproduction, is proof of the position that the constant development in man is not a warrant for their constant use. ”

“ In the simplest forms of conjugation the functions of both parents appear to be identical, but in organisms which are a little more specialized we find male and female reproductive bodies, and the offspring is the result of the union of the male element of one individual with the female element of another; that is, we have true sexual reproduction in its simplest form. ”

“ Problems of Reproduction.—The problems of reproduction are extremely broad, involving not only the immediate questions of individual reproduction, but also those broader and deeper ones which relate to heredity. ”

“ Sexual reproduction does not essentially differ, as we have seen, from budding or self-division, and these processes graduate through the repair of injuries into ordinary development and growth; it might therefore be expected that every character would be as regularly transmitted by all the methods of reproduction as by continued growth. ”

“ Yet the whole process of sexual reproduction undoubtedly has, as its origin and explanation, the fusion in the first cell of the new generation from which all the rest will arise, of the material of two distinct individuals. ”

“ CHAPTER IVToC THE SEXUAL APPETITE
If we sum up the three preceding chapters we arrive at the philosophical conclusion that reproduction depends on the general natural tendency of all living beings to multiply indefinitely. Fission and sexual reproduction arise from the simple fact that the growth of each individual is necessarily limited in space as well as time. Reproduction is thus destined to assure the continuation of life; the individual dies but is perpetuated in his progeny. We do not know why the crossing of individuals is rendered necessary by the phenomenon of conjugation. ”

“ These reproductive cells contain the germinal material or germ-plasm, and this again, in its specific molecular structure, is the bearer of the hereditary tendencies of the organisms from which the reproductive cells originate. Thus in sexual reproduction two hereditary tendencies are in a sense intermingled. In this mingling, I see the cause of the hereditary individual characteristics; and in the production of these characters, the task of sexual reproduction. It has to supply the material for the individual differences from which selection produces new species. ”

“ But when I maintain that the meaning of sexual reproduction is to render possible the transformation of the higher organisms by means of natural selection, such a statement is not equivalent to the assertion that sexual reproduction originally came into existence in order to achieve this end. ”

“ Success in the attainment of reproduction depends upon the successful discharge of the sexual function; and the discharge of the sexual function depends primarily upon an individual of one sex coming into contact with one of the opposite sex at the appropriate season and when its appropriate organic condition arises. ”

“ In all the different forms of a-sexual reproduction, from fission to parthenogenesis, the female may in this sense be said to exist alone and perform all the functions of life, including reproduction. ”

“ The reproductive faculty of the males and the females of this distinct and peculiar animal is limited to the generative reproduction of individuals of the same type, by a sexual union of two individuals of that type. ”

“ Sexual and asexual reproduction are thus seen not to differ essentially; and we have already shown that asexual reproduction, the power of re-growth and development are all parts of one and the same great law. ”

“ And so, when we come to consider the question of reproduction in the higher forms of life, we are apt to focus attention too much upon the sexual function and too little upon the contributory factors, the failure of any one of which would mean failure of the whole. ”

“ It clearly brings before us the difference between Nature's earlier and later methods, the ever growing preference for quality of offspring over quantity.
It has been necessary to touch on the wider aspects of reproduction in Nature, even when our main concern is with particular aspects of reproduction in man, for unless we understand the progressive tendency of reproduction in Nature, we shall probably fail to understand it in man. With these preliminary observations, we may now take up the question as it affects man. ”

“ II. That sex is determined in the moment of fertilization by the reciprocal interaction of male and female, of zoösperm and ovum. One variant of this theory maintains that each reproductive element strives for the reproduction of its own sex; that a struggle takes place and that the victor in the contest stamps its own sex-likeness upon the fertilized product. ”

“ It is an interesting physiological fact that, while the growth and development of the individual are rapidly progressing, the reproductive powers remain almost inactive, and that the commencement of reproduction not only indicates an arrest of growth, but, in a great measure, contributes toward it. ”

“ A limited number of observations and experiments have been made to test out the correlation between sexual reproduction and variation, but they have so far been too few or too inconclusive to enable us to come to a satisfactory conclusion. ”

“ Comparison shows that there is, on the one hand, no essential distinction between ordinary growth and reproduction by budding, and, on the other hand, none except the necessity for impregnation to distinguish asexual from sexual reproduction. ”

“ These phenomena are (1) parthenogenesis; (2) oviparous and viviparous reproduction; (3) the production of generations of individuals in which the sexes are very unequally represented, males being frequently entirely absent; (4) the production of individuals differing as to the acquirement of wings, some remaining entirely apterous, while others go on to the winged form; (5) the production of individuals of the same sex with different sexual organs, and distinctions in the very early (but not the earliest) stages of the formation of the individual ”

“ We know of female and male individuals, among both animals and plants: their differences consist essentially in the fact that they produce different kinds of reproductive cells; in part they are of a secondary nature, being adaptations of the organism to the functions of reproduction; they are intended to attract the other sex, or to ensure the meeting of the two kinds of reproductive cells, or to enable the fertilized egg to develope and sometimes to guide the development of the offspring until it has reached a certain period of growth. ”

“ These higher types of animal life have been provided by nature with special organs of reproduction and reproductive glands whose secretions, when they are projected from the body under certain conditions, reproduce themselves, and increase and develop in such wise that the living organism from which they proceed is reproduced in practically its identical form. ”

“ Every one has a summary answer to this question in the statement, that all these short-lived individuals reproduce themselves, and thus maintain their kinds. But the modes of reproduction are so varied, the changes some animals undergo during their growth so extraordinary, the phenomena accompanying these changes so startling, that, in the pursuit of the subject, a new and independent science—that of Embryology—has grown up, of the utmost importance in the present state of our knowledge. ”

“ We have observed that, according to naturalists, the earliest forms of life which appeared on the earth were androgynous or hermaphrodite, that the two elements necessary for reproduction were originally confined within one and the same individual within which were carried on all the functions of reproduction. Later, however, a division of labour arose, and these two original functions became detached, after which time the reproductive processes were carried on only through the commingling of elements prepared by, or developed within, two separate and distinct individuals. ”

“ In order that reproduction may continue without interruption, these writers believe that a rejuvenescence of the living substance is necessary, that the clock-work of reproduction must be wound up afresh; and they recognize such a rejuvenescence in sexual reproduction and in conjugation, or in other words in the fusion of two cells, whether in the form of germ-cells or of two unicellular organisms. ”

“ That in plants, and animals as well, that are actively occupied in vegetative, physiological, pathological, or other efforts which are antagonistic or complementary to the office of reproduction, the proportion of females born during such times is greater than where the plant or animal has reached full developmental maturity and growth, is in good health, and is occupied principally in the process of reproduction. ”

“ Gradually we have come to understand the whole matter of reproduction very much better. Minute and careful dissections of rabbits, of dogs and cats, of animals slaughtered for food, with occasional post-mortem examinations of human beings in various stages of the development of the young, leave us no longer in doubt concerning the main features of the process. The better we come to understand it the more clearly it becomes evident that in the development of the mammals we have no new procedure, but, as in so many other activities, new developments of an old process. ”

“ Hence, philosophically speaking, the most general view of reproduction coincides exclusively with this nutritive process—its type is discoverable in vegetable organisms, and the varying phenomena of animal life involved in the conjugation of reproductive cells may be considered apart. ”

“ The Reproductive Elements.—As has been previously observed, in all except the very lowest forms of life, two elements are necessary to the production of a new individual, or a reproduction of the species—a male element and a female element. The special organs by means of which these elements are produced, brought together, and developed into the new individual in a more or less perfect state, are termed sexual organs, as we have already seen. ”

“ It must include all the developmental processes which take place as a result of sexual reproduction. A man at birth, when he ceases to be an embryo, has still many changes besides those of simple growth to pass through. ”

“ The mode of reproduction is by ova, which are sometimes vivified in the body of the parent, and not only do they give birth by ejection from the mouth of a numerous progeny, but actual divisions of the body may be made, and each division will acquire completeness. ”

“ To those who find it difficult to assimilate this somewhat startling idea, we now present a brief statement of the infinitely greater facility toward reproduction manifested by living creatures lacking in sex-differentiation as compared with those possessing it. ”

“ Before bringing our subject to its practical bearings, it seems necessary that the reader should divest his mind of any vestiges of the glamour of superstition which has so long been around the reproductive process. Darwin succeeds in this with admirable directness by putting reproduction simply as a process of growth. ”

“ As a term in biology or physiology, generation is synonymous with the Gr. βιογένεσις and the Ger. Zeugung, and may comprehend the whole history of the first origin and continued reproduction of living bodies, whether plants or animals; but it is frequently restricted to the sexual reproduction of animals. The subject may be divided into the following branches, viz.: (1) the first origin of life and living beings, (2) non-sexual or agamic reproduction, and (3) gamic or sexual reproduction. ”