But we must not forget--and, indeed, one should not enter on the consideration of this subject without a knowledge of the fact--that vast numbers of animals and plants reproduce themselves "asexually," as it is termed, namely, by breaking-off or separating buds, branches, or other good solid bits of their structure which, when thus separated, are capable of individual life and growth. Thus plants very largely multiply, using this method in addition to the sexual method of egg-cells and sperm-cells. One may take "cuttings" from plants and rear them, and plants also "cut" or detach such bits themselves, in the form of runners, of dividing bulbs, of bulbules, and such reproductive growths seen on the lily, on the viviparous, alpine grass, and many other plants. Even a bit cut off from the leaf of a plant (for instance, a begonia) will sprout, root itself, and grow into a completely formed and healthy individual. Animals, too, such as polyps or zoophytes, and many beautiful and elaborate worms, multiply by "fission," dividing into two or more parts, each of which becomes a complete animal. This process is not seen in any fish, amphibian, reptile, bird, or mammal, nor in molluscs, nor in insects, crustaceans, myriapods, and arachnids (spiders and scorpions). It is almost wholly confined to lower animals (worms and polyps) and to plants, and hence is often called "vegetative reproduction." The most remarkable case of its appearance among higher forms is that of the marine Ascidians, or tunicates--close allies of the true vertebrates--where reproduction by budding and the formation of wonderfully elaborate star-like forms produced by budding and the cohesion of the budded individuals as one composite individual are well known. Their beautiful shapes and colours have been reproduced in hundreds of exquisite pictures by our great artist-naturalists. We thus have to recognise that there are two distinct kinds of reproduction in living things. One is "asexual," by means of division or separation of large or special masses of their existence, made up of ordinary tissue cells. Co-existing with this, often in the same individuals, is the other method, the "sexual," by means of detached egg-cells and sperm-cells which are thrown off from the parents, and do not (except in rare instances) proceed to develop unless the egg-cell is "fertilised" by the fusion with it of a sperm-cell.