The chemical forces may be considered under the subdivisions aquatic, atmospheric, food, and soil. In class A may also be included (3) vital or organic force in so far as this is concerned with the interaction of organisms, and it is seen thus to link the two great classes. The second class (B) includes (1) physiological and (2) psychical forces. Prominent among the former, as causes of modification, are worthy of mention those connected with genesis itself: as heredity, physiological selection, sexual selection, hybridity, primogenital selection, and what I would call sexual differentiation, and philoprogeneity. Among the latter may be included use and disuse, individual effort, etc.; and last, but not least, the emotions.
Now, with the limited definition given to natural selection, all the forces in class A act independently of it, while the rest are more or less fully aids to its action. Time will not permit of much detailed consideration of the physical and chemical forces. Nor is such consideration necessary; for their influence, as Darwin well remarked, is obvious. Fundamentally, they must needs limit and control all manifestations of life, of which indeed, on evolutional grounds, they are the material basis. Change of physical environment may affect function first and chiefly, but this involves change of form and structure which are integrated by heredity. The surface of the earth and the waters upon it and the atmosphere above it have necessarily conditioned the chief modes of animal locomotion, as swimming, flying, crawling, and walking, while the five great classes of vertebrates find the explanation of their structure, as J. B. Steere pointed out at the Ann Arbor meeting, in the conditions of life in water, in shallows, in the air, on land, and on trees and rocks.
External Conditions.—By external conditions, or environment, we include all influences on organisms which act from without, and in carefully considering them we shall find it difficult to draw the line between those which are really external and independent of any motive or inherent tendency in the organism, and those which are not. Hence, the general term "external conditions" is resolvable into various minor factors. Considering the influences as a whole, we find that in the 1844 essay, or sketch, Darwin gave more weight to them as producing variations, and as modifying habit, than he did in the "Origin"; yet we all know that he felt convinced, when this work was first issued, that natural selection was the main, though not the exclusive, means of modification. Before his death, he was again led to attach greater
- I am well aware that this term is much tabooed among a certain class of the more materialistic evolutionists, but I use it here for want of a better, and because, as an expression of one form of manifestation of force, it has as much a classificatory value as physical or psychical.