The marked difference in appearance which exists between the young at birth and their parents is another striking fact in the study of reproduc tion. In some species, especially among the invertebrates, this difference amounts to a degree which has made it difficult for scientists to identify the young as the actual progeny of their parents ; and at times it has even lent colour to the hypothesis of spontaneous generation—that is, the produc tion of living beings from inanimate material ; in short, progeny without parents. This hypothesis, however, has never been established in a single instance ; and a number of the organisms which were supposed by some to be so generated have been shown, by acute and painstaking experiment and observation, to belong to a definite cycle, often long and involved, which ultimately resulted in the development of organisms exactly like the parents from which the apparently unrelated progeny actually sprung. The difficulty of keeping such cycles under continuous observation is what makes them seem mysterious. In reality, more familiar ones are equally remarkable. For instance, if we were unable to see the process by which the caterpillar grows to resemble its parent, the butterfly, we might be very loath to believe in their identity. It is because every child can catch a caterpillar, and watch the spinning of the cocoon and the emergence of the butterfly, that the process seems less remarkable than one which goes on among the parasites in the tissues of a pig.
In the vertebrates, the difference between the young and the adult is less surprising, and even the form of the embryo presents some analogy to that of the adult. Yet even the new-born child is quite sufficiently different from the mature man ; and it is probable that, if we could view him with an eye wholly uninfluenced by knowledge of what he is going to be, we might realise more actively that the reproduction of form which marks a species is brought about through a long-continued series of changes. This series is in the form of a circuit, as it were, and any point in that circuit might be taken as a starting-point ; but it is perhaps most reasonable to consider it as starting with the egg, and culminating with the production of the egg again.
In all the higher plants and animals we find sexual generation ; that is, generation resulting from a union of the two sexes, male and female. Each sex is characterised by certain organs peculiar to itself, which give rise to a certain organised product. This product, when united with the
corresponding product of the other sex, results in the formation of a new individual of the same species. Thus, the female organism produces the egg or germ, and the male organism the sperm or spermatic fluid. When the egg is fecundated by the spermatic fluid, it becomes capable of developing into the young plant or animal. In flowering plants the male product is represented by the pollen which fecundates the germ and enables it to develop into the fruit. In many species of plants the male and female organs are found in the same blossom ; in others, they occur in different blossoms on the same plant ; and in still others, they are found on distinct plants. In most species of animals the female organs, or ovaries, which supply the egg are found in one individual, and the male organs, or testicles, which supply the spermatic fluid are found in another. In some of the invertebrates, however, notably the snail and the earthworm, ovaries and testicles are found in the same individual. Even in these species, however, the union of two individuals is necessary to reproduction, as the eggs of one are fertilised by the seminal fluid of another.
In the vertebrates the sexes are distinct, each possessing its characteristic generative organs, generally supplemented by accessory organs of generation which assist in the process, and occasion a difference, more or less marked, in bodily form. The various species differ in the structure of the male and female organs, the manner in which the sexual products are formed and discharged, and their union in the act of fecundation. In most fish the eggs are deposited by the female, and then impregnated by the male. In the frog the eggs are impregnated at the moment when they are expelled. In serpents, lizards, and turtles the eggs are fecundated in the generative passages of the female, some species laying the eggs immediately, and others retaining them until the embryo is partly developed. In birds the fecundation takes place as soon as the egg is discharged from the ovary. This is true also in man and in mammals, but in them the fertilised egg is retained in the body of the female during the entire period of its develop ment. A further discussion of the generative organs in man will be found in the INTRODUCTORY CHAPTERS (pp. 164-169). See also the articles PARTURITION ; PREGNANCY ; SEXUAL DESIRE.