Naturalists are constantly speaking of “related species,” of the “affinity” of a genus or other group, and of “family resemblance,”—­vaguely conscious that these terms of kinship are something more than mere metaphors, but unaware of the grounds of their aptness.  Mr. Darwin assures them that they have been talking derivative doctrine all their lives without knowing it.

If it is difficult and in some cases practically impossible to fix the limits of species, it is still more so to fix those of genera; and those of tribes and families are still less susceptible of exact natural circumscription.  Intermediate forms occur, connecting one group with another in a manner sadly perplexing to systematists, except to those who have ceased to expect absolute limitations in Nature.  All this blending could hardly fail to suggest a former material connection among allied forms, such as that which an hypothesis of derivation demands.

Here it would not be amiss to consider the general principle of gradation throughout organic Nature,—­a principle which answers in a general way to the law of continuity in the inorganic world, or rather is so analogous to it that both may fairly be expressed by the Leibnitzian axiom, Natura non agit saltatim.  As an axiom or philosophical principle, used to test modal laws or hypotheses, this in strictness belongs only to physics.  In the investigation of Nature at large, at least in the organic world, nobody would undertake to apply this principle as a test of the validity of any theory or supposed law.  But naturalists of enlarged views will not fail to infer the principle from the phenomena they investigate,—­to perceive that the rule holds, under due qualifications and altered forms, throughout the realm of Nature; although we do not suppose that Nature in the organic world makes no distinct steps, but only short and serial steps,—­not infinitely fine gradations, but no long leaps, or few of them.

To glance at a few illustrations out of many that present themselves.  It would be thought that the distinction between the two organic kingdoms was broad and absolute.  Plants and animals belong to two very different categories, fulfil opposite offices, and, as to the mass of them, are so unlike that the difficulty of the ordinary observer would be to find points of comparison.  Without entering into details, which would fill an article, we may safely say that the difficulty with the naturalist is all the other way,—­that all these broad differences vanish one by one as we approach the lower confines of the two kingdoms, and that no absolute distinction whatever is now known between them.  It is quite possible that the same organism may be both vegetable and animal, or may be first the one and then the other.  If some organisms may be said to be at first vegetables and then animals, others, like the spores and other reproductive bodies of many of the lower Algae, may equally claim to have first