REPRODUCTION, the term applied to the whole process whereby life is con tinued from generation to generation. One of the characteristics of life is its continuity; the races of animals and the orders of plants live on without marked change for centuries; by slow modifica tions they may be enriched or impover ished, increased or thinned, but there is no breach of continuity. All the forms of life seem to evolutionists like twigs on one many-branched tree; they are gen etically related by near or distant bonds of kinship, and in a very real sense each generation is continuous with those which come before and after it.
Modes of Reproduction.—Separated fragments of a sponge or cuttings from the rose, the buds of a hydra, or the bulb ils of a lily, the eggs of a bird, and the seeds of plants are alike able to grow into new organisms; and thus we see that the common fact about all kinds of reproduction is that parts of one organ ism are separated to form or to help to form new lives. In many cases what is separated from the parent life is simply part of its body, an overgrowth or a definite bud, which, being set free, is able to reproduce the whole of which it is a representative sample. This is called asexual reproduction. In most cases, however, the parents give origin to spe cial reproductive elements—egg cells and male cells—which combine and are to gether able to grow into a new life. This is called sexual reproduction.
The simplest forms of reproduction are found among the single-celled plants and animals. There we may find an or ganism like Schizogenes, multiplying by breakage, reproducing by rupture, pre sumably when the cell has overgrown its normal size; in others numerous buds are liberated at once, as in Arcella and Pelomyxa; in many, familiarly in the yeast plant, one bud is formed at a time; in most the cell divides into two or many daughter cells. The cast-off arm of a starfish may regrow the entire animal with a readiness that suggests a habit; some kinds of worms (e. g., Nemerteans)
break into pieces, each of which is able to regrow the whole; large pieces of a sea anemone or of a sponge are some times separated off and form new organ isms.
But the usual mode of asexual repro duction is by the formation of definite buds. When these buds remain continu ous, colonial organisms result, like many sponges, most hydroids, Siphonophora like the Portuguese man-of-war, many corals, almost all the Polyzoa, and many Tunicates. The runners of a strawberry and the suckers which grow around a rose bush illustrate the same state. But in a few plants, like the liverwort and the tiger lily, a kind of bud may be de tached, and thus begin a new life. It is among animals, however, that the libera tion of buds is best illustrated, for this mode of reproduction occurs in hydra and many hydroids, in some "worms," and in Polyzoa, and even in animals as highly organized as Tunicates. Budding is usually exhibited by comparatively simple and by sedentary animals, and seems indeed to be natural to vegetative organisms. Budding is only possible when the organism is not very highly dif ferentiated, or when part of the body re tains many indifferent units; moreover, it is an expensive way of securing the continuance of generation, and is without the advantage to the species which un doubtedly results from the mingling of two life-currents in sexual reproducton.
Sexual reproduction in its fully dif ferentiated form involves (a) the dis tinctness of two parent organisms, (b) the formation of two different kinds of reproductive elements — e. g. spermato zoa produced by the male and ova by the female, and (c) the fertilization of the egg cell by a male element. Moreover the process of sexual reproduction also includes the sexual union of the two parents, or other ways in which fertili zation is secured, while in some cases the fertilized ovum develops in organic re lation with the innther organism, from which it is eventually separated as an embryo.