Darwin’s theory is therefore not what his opponents frequently represent it as being—an unwarranted hypothesis taken up at random. It is not for zoologists or botanists to accept or reject this as an explanatory theory, as they please; they are rather compelled and obliged to accept it, according to the general principle observed in all natural sciences, that we must accept and retain for the explanation of phenomena any theory which, though it has only a feeble basis, is compatible with the actual facts—until it is replaced by a better one. If we do not adopt it, we renounce a scientific explanation of phenomena, and this is, in fact, the position which many biologists still maintain. They look upon the whole domain of animate nature as a perfect mystery, and upon the origin of animals and plants, the phenomena of their development and affinities, as quite inexplicable and miraculous; in fact, they will not allow that there _can_ be a true understanding of them. Those opponents of Darwin who do not exactly wish to renounce a scientific explanation are in the habit of saying, “Darwin’s theory of the common origin of the different species is only _one_ hypothesis; we oppose to it _another_, the hypothesis that the individual animal and vegetable species have not developed one from another by descent, but that they have come into existence independently of one another, by a still undiscovered law of nature.” But as long as it is not shown how this coming into existence is to be conceived of, and what that “law of nature” is—as long as not even _probable_ grounds of explanation can be brought forward to account for the independent coming into existence of animal and vegetable species, so long this counter-hypothesis is in fact no hypothesis, but an empty unmeaning phrase. Darwin’s theory ought, moreover, not to be called an hypothesis. For a scientific hypothesis is a supposition, postulating the existence of unknown properties or motional phenomena of natural bodies, which properties have not as yet been observed by the experience of the senses. But Darwin’s theory does not assume such unknown conditions; it is based upon general properties of organisms that have long been recognized, and—as has been remarked—it is the exceedingly ingenious and comprehensive combination of a number of phenomena which had hitherto stood isolated, which gives the theory its extraordinarily great and intrinsic value. By it we are for the first time in a position to demonstrate an active cause for all the known morphological phenomena in the animal and vegetable kingdoms; and, in fact, this cause is always one and the same, viz., the alternate action of Adaptation and Inheritance, therefore a physiological, that is, a physico-chemical or mechanical, relationship. For these reasons the acceptance of the Doctrine of Filiation, as mechanically established by Darwin, is a binding and unavoidable necessity for the whole domain of zoology and botany.