THE LAW OF CONTINUITY. 35 



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 reser- 

 voir 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, wax- 

 cells, and so on, are not rigidly invariable, but are always more or 

 less adapted to circumstances. Glass rods have been placed in a bee- 

 hive, and the little workers to avoid them have sprung all sorts of but- 

 tresses and arches, such probably as neither they nor any of their pro- 

 genitors 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 zoological classification yet produced. 



Profs. Huxley and Haeckel describe a tree of life : the main branch- 

 es 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 re- 

 course 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 for- 

 midable difficulty in the way of the reception of the principle of con- 

 tinuity 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 ex- 

 pertness of this kind, which we must mark is very apt to run in a nar- 

 row 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. 



