As before remarked, reproduction is a function common to all animals and to all plants. Every organized being has the power to reproduce itself, or to produce, or aid in producing, other individuals like itself. It is by means of this function that plants and animals increase or multiply. When we consider the great diversity of characters illustrated in animal and vegetable life, and the infinite variety of conditions and circumstances under which organized creatures exist, it is not surprising that modes of reproduction should also present great diversity both in general character and in detail. We shall find it both interesting and instructive to consider some of the many different modes of reproduction, or generation, observed in different classes of living beings, previous to entering upon the specific study of reproduction in man. Before doing thus, however, let us give brief attention to a theoretical form of generation, which cannot be called reproduction, known as Spontaneous Generation.--By this term is meant the supposed formation of living creatures directly from dead matter without the intervention of other living organisms. The theory is, in substance, an old one. The ancients supposed that the frogs and other small reptiles so abundant in the vicinity of slimy pools and stagnant marshes, were generated spontaneously from the mud and slime in which they lived. This theory was, of course, abandoned when the natural history of reptiles became known. For several thousand years the belief was still held that maggots found in decaying meat were produced spontaneously; but it was discovered, centuries ago, that maggots are not formed if the flesh is protected from flies, since they are the larvae, or young, of a species of this insect. A relic of the ancient belief in spontaneous generation is still found in the supposition that horse-hair snakes, so-called, are really formed from the hairs of horses. This belief is quite common, but science long ago exposed its falsity. When the microscope was discovered it revealed a whole new world of infinitesimal beings which were at first supposed to be of spontaneous origin; but careful scientific investigation has shown that even these mere specks of life are not independent of parentage. M. Pasteur and, more recently, Prof. Tyndall, with many other distinguished scientists, have demonstrated this fact beyond all reasonable chance for question. It is, then, an established law that _every living organism originates with some previously existing living being or beings_.