The tendrils of vines find points for attachment an inch or two from their stems; in cellars and caverns the feeble sprouts grow toward the light which they seem to feel is their life.
Is not all this conformable to the law by which motion takes the path of least resistance, as in the case of the waters of a broken reservoir descending to a valley by the shortest channel; or discharges of electricity harmlessly betaking themselves to the earth through a purposely-exposed conductor?
Instinct, especially in insects, borders on and at times invades the higher realm of intelligence. The shapes of birds'-nests, waxcells, and so on, are not rigidly invariable, but are always more or less adapted to circumstances. Glass rods have been placed in a beehive, and the little workers to avoid them have sprung all sorts of buttresses and arches, such probably as neither they nor any of their progenitors ever undertook before.
Natural history, in the discussions which have recently shaken the world, illustrates how difficult, if not impossible, is the task of trying to draw lines of demarkation, hard and fast, in Nature. The arguments pro and con as to what constitutes a true species might be gathered into a very bulky volume, and the end of the discussion is not yet.
The probability of truth, on the side of those naturalists who affirm the principle of continuity as explaining the genesis of species, has been strengthened by that principle being made the basis of the best method of zoölogical classification yet produced.
Profs. Huxley and Haeckel describe a tree of life: the main branches of it are the great classes; the divergent limbs, the families; and the minor branches, the species. The wide gaps between the groups of organisms now extant are in considerable measure bridged by recourse to fossils, and the suggestions of embryology—which science studies the phases an animal passes through from conception to birth, and observes the affiliations indicated in antenatal history.
As the gulfs existing between living things present the most formidable difficulty in the way of the reception of the principle of continuity in its broadest claims, it may be admissible here to present some of the explanations given by Lyell and others to account for the fact that so many links of genetic connection are missing. It is most important to a species that it should preserve and intensify some definite method of subsistence—a habit of diving, climbing, swimming, digging, or of catching some particular prey, or finding and living on some special plant. There is a natural premium set upon some expertness of this kind, which we must mark is very apt to run in a narrow groove; and there is a yet greater reward for any new expertness, the occupying of a new field of animal possibility, or an adaptation to circumstances changed by the great forces of Nature—as in the mighty revolutions brought about by astronomical and geological causes.