Asexual reproduction

“ Asexual reproduction.—Formation of the animal colony.—Limits of asexual reproduction.—Coupling.—Birth of the sexes.—Hermaphrodism and parthenogenesis.—Chemical fecundation.—Universality of parthenogenesis.
The primitive mode of reproduction is asexual, or what one will so consider provisorily, in comparison with more complex mechanism. In the first living forms there are neither sexual organs nor differentiated sexual elements. The animal reproduces itself by scissiparity or by budding ”

“ Biologists tell us that the meaning or purpose of sexual reproduction is to bring about greater organic variation. Now variation, as we have seen, is the raw material upon which natural selection acts to create the higher types. The immense superiority of sexual reproduction over asexual reproduction is due to the fact that it multiplies so greatly the elements of heredity in each new organism, for under sexual reproduction every new organism has two parents, four grandparents, and so on, each of which perhaps contributes something to its heredity. ”

“ In these the multiplication of individuals takes place by unsexual reproduction, which takes the form of cleavage, budding, or spore-formation. The copulation of two coalescing cells, which in these cases often precedes the reproduction, cannot be regarded as a sexual act unless the two copulating plastids differ in size or structure. On the other hand, sexual reproduction is the general rule with all the higher organisms, both animal and plant; very rarely do we find asexual reproduction among them. ”

“ Either sex may transmit characters which in themselves are necessarily latent, as, for instance, a bull may transmit a good milking strain. In forms of asexual reproduction, such as division, budding, propagation by slips and so forth, every character of the parent may appear in the descendant, and apparently even in the descendants produced from that descendant by the ordinary sexual processes. ”

“ 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. ”

“ Thus we have shown the easy gradation, and therefore the probable derivation, of the highest mode of sexual reproduction—the unisexual—from the ordinary processes of growth, through the different grades of asexual and bisexual reproduction. The derivation of different modes of sexual reproduction from each other will not, I think, be questioned. Still clearer is the fact that non-sexual reproduction is but a modification of the ordinary process of growth. The only place where there is any gap is between the asexual and the sexual modes. ”

“ In close connection with the foregoing we have the fact, reported by Maupas,16 that certain Infusorians are capable of reproducing asexually for a number of generations, but that, unless the individuals are sexually fertilized by crossing with unrelated forms of the same species, they finally exhibit all the signs of senile degeneration, ending in death.17 After sexual conjugation there was an access of vitality, and the asexual reproduction proceeded as before. ”

“ Asexual passes into sexual reproduction by a series of gradual transitions that are no longer a mystery; they have been well discriminated by Professor Le Conte. The mechanical rupture of simple organisms is followed by budding on any part, that by budding on a special part, that by the formation of an internal organ, which in a still more advanced stage generates at once male and female cells; differentiated sexual elements are next produced by independent sexual organs, which are ultimately assigned to separate individuals. Asexuality passes through bisexuality into unisexuality. ”

“ 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. ”

“ If the asexual method of reproduction alone existed, there would, so far as our knowledge at present extends, be no increase in the number of kinds of organism: no new individuality could arise. The first establishment of a new kind of individual by the sexual process is effected in a very similar manner in all Metazoa. The parent produces by a process of unequal fission, which takes place at a part of the body called the reproductive gland, a small living organism called the reproductive cell. ”

“ We see everywhere that sexual union between quite distinct animal species gives no result. At the most, certain closely allied species, such as the ass and the horse, the rabbit and the hare, give progeny which are themselves sterile (mules, etc.) . The feebleness and sterility of hybrids derived from widely separated races or nearly allied different species proves the deficiency in vital force of the offspring of fundamentally dissimilar procreators. But, on the other hand, the dangers of continuous consanguineous reproduction are no less evident. ”

“ In cultures of the latter there occur very rare supplemental males which appear in no sense degenerate but as fit for reproduction as the males of the bisexual species. Though possessing a complete copulatory apparatus and producing large quantities of spermatozoa, they have lost their sexual instinct and play no part in the economy of the species. These “psychically decadent” individuals appear to represent the entire male sex of a bisexual species, and become unnecessary owing to the grafting of hermaphroditism on the female sex. ”

“ The male, however, is much smaller, and far less developed than the female. In some of these species, germ-cells, or eggs, are found, which do not require fecundation for reproduction or development, so that they belong to the non-sexual class.
The third variety of hermaphrodites embraces those animals in which the male organs are so disposed as not to fecundate [pg 201] the ova of the same body, but require the co-operation of two individuals, notwithstanding the co-existence in each of the organs of both sexes. Each in turn impregnates the other. ”

“ The lower forms of animal and plant life often reproduce themselves without the aid of sex, and it has even been argued that reproduction and sex are directly antagonistic, that active propagation is always checked when sexual differentiation is established. "The impression one gains of sexuality," remarks Professor Coulter, foremost of American botanists, "is that it represents reproduction under peculiar difficulties." ”

“ 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. ”

“ Now, while Spencer was probably right in saying that fertilisation promotes change, we cannot think that he succeeded in finding what he was seeking, namely a primary physiological reason why sexual reproduction should occur. It may be pointed out that it is only in a limited sense that sperm-cells or egg-cells can be spoken of as in a state of "quiescence," and that it is only in a limited sense that the organism which has finished growing and is beginning to be sexual can be spoken of as in a state of general or molecular equilibrium. ”

“ The best equipped males will thus crowd out the less well-equipped in the matter of reproduction, and thus the relevant characters will be increased and perfected through sexual selection. It is, of course, a necessary assumption that these secondary sexual characters may be transmitted to the female, although perhaps in rudimentary form. ”

“ In a given mating, we must take into consideration such factors as impediments to coitus, as well as those numerous agencies in the female which may interfere with the union of sperm and ovum, or with the successful implantation of the fertilized egg in the uterus, and its growth and development there till normal parturition takes place. Successful reproduction depends upon the mating of sexually sound females to equally sound males. ”

“ We are thus led to believe that sexuality is a means for infusing vigour into the offspring by the coalescence of differentiated elements, an advantage which could not follow if reproductions were entirely asexual. ”

“ It is a common belief that the original purpose of sex is to ensure reproduction, whereas fundamentally it is not necessary to propagation at all. It is perfectly true, of course, that [36] in the majority of animals, and also in many plants, an individual life begins in the union of two minute elements, the mother egg-cell and the sperm father-cell. But this is not the earliest stage, and below these higher forms we find a great world of life reproducing without this sex-process by simple separation and growth. ”

“ From the several foregoing considerations we may conclude that the difference between sexual and asexual generation is not nearly so great as at first appears; the chief difference being that an ovule cannot continue to live and to be fully developed unless it unites with the male element ”

“ By this method there is of necessity great waste in the production of offspring, many thousands of eggs are never fertilised. The union of the sexual cells must be something more than haphazard for further development. There must be some reason inherent in the female or male inducing to the act of reproduction. In other words, there must be a psychic interest preceding the sex act. In this way a higher grade is reached when the presence of one sex attracts the other. Gradually the female and the male begin to associate in pairs. ”

“ 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. ”

“ 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 feature common to all ordinary forms of asexual multiplication is that the reproductive process is independent of sex; what starts the new life is the half, or a liberated portion of the single parent cell. It will be readily seen that by this process the offspring are identical with the parent. Life continues, but it continues unchanged. Thus the power of growth is restricted within extremely narrow limits. Any further development required a new process. With the life-force pushing in all directions every possible process would be tried. ”

“ We believe the answer to be that sexual reproduction is an adaptive process securing the benefits of amphimixis, for in amphimixis and in the changes preparatory to it, there is an important source of variation. In one of his essays Weismann wrote as follows:—
"Sexual reproduction is well known to consist in the fusion of two contrasted reproductive cells, or perhaps even in the fusion of their nuclei alone. ”

“ In his opinion the determination of the sex of the embryo depends neither upon the quality of the ovum nor upon that of the spermatozoon, nor again upon the reciprocal influences exerted by ovum and spermatozoon on one another. During the earliest time after fertilization the embryo is sexually neutral and only later, as a result of some new influence acting upon it, does it receive an impulse toward the formation either of the male or of the female sex. ”

“ Galton supposes, in sexual reproduction, where two parents contribute germs to the embryo the chance of deficiency of any of the necessary germs is greatly diminished. Darwin's "very different view of the meaning or cause of sexuality" is no doubt that given in "Cross and Self Fertilisation"—i.e., that sexuality is equivalent to changed conditions, that the parents are not representative of different sexes, but of different conditions of life. ”

“ It is certain that so difficult a problem as that of the significance of sexual reproduction requires much more investigation. Darwin was anything but dogmatic and always ready to alter an opinion when it was not based on definite proof: he wrote, "But the veil of secrecy is as yet far from lifted; nor will it be, until we can say why it is beneficial that the sexual elements should be differentiated to a certain extent, and why, if the differentiation be carried still further, injury follows." ”

“ The offspring will thus be the result of the unfolding of a bit of germ plasm which will be different from that from which either of its parents developed, and these differences will result in congenital variations. Sexual reproduction thus results in congenital variations; and if congenital variations are necessary for the evolution of the living machine—and we shall soon see reason for believing that they are—we find that sexual reproduction is a device adopted for bringing out such congenital variations. ”