Page:Popular Science Monthly Volume 8.djvu/619

Rh doctrine which was supposed to have been disproved two hundred years since, and which reappeared in the last century only to be assailed with Voltaire's sarcasms. I mean the theory of spontaneous generation, so called—a self-contradictory phrase, by which it was intended to assert that organisms are produced out-and-out without the aid of parents resembling them. While admitting that generation, sexual or asexual, is the mode of reproduction found among animals possessed of complex structure, the partisans of spontaneous generation held, on the strength of their experiments, that certain very low organisms might be developed spontaneously, without specific germs, in infusions of organic substances. But though in this dispute experiment has given no definitive verdict—nor, indeed, was such verdict to be expected—still, all the probabilities are on the side of those who assert the universality of generation by means of germs developed in the parents; and, in the absence of experimental demonstration, we are not without theoretic arguments against the spontaneous generation of the comparatively high organisms developed in infusions. If this doctrine is to be retained, it is not for the purpose of explaining the formation of organisms, a thing well enough explained without it, but in order to account for the production of really primitive living things—i. e., for the appearance of life in a fraction of organic substance, whether this is still possible in our day, or whether it was possible only at a time when, under conditions unknown to us, organic substance originated upon the earth. Thus stated, the question does not depend on experimentation; it becomes a mere exercise of the imagination, and the result is valueless.

Whatever is to be thought of the theory of the beginnings of life, one or more first living beings having appeared upon the earth, after the latter had become capable of supporting them, the question arises as to the transition from the primitive simplicity to the enormous degree of variety now existing. Here we have the problem of the origin of species, which is solved by the theory of descent, sometimes denominated transformism. The old conception of living Nature as an infinitely varied assemblage of organisms which faithfully copy certain types, all of whose parts are governed by the law of final causes, in our time gives way, not without a fierce struggle, before a new conception, which represents living Nature as an infinitely varied assemblage of organisms which are ever varying under the influence of external circumstances, while under the influence of heredity they tend to fix in a type the results of previous variations. At one time we have, as in breeding, artificial selection; at another time, as among people who have not yet discovered the laws of breeding, a selection that, though unsystematic, is nonetheless real; finally in Nature, without human intervention, a selection based simply on the conditions of existence. In natural selection, the action of which is by far the most general and powerful, the fixing of variations results from adaptation