6>iV THE CAUSES OF VxVRIATION. 487 



The chemical forces may be considered under the subdivisions 

 aquatic, atmospheric, food, and soil. In class A may also be in- 

 cluded (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) plujsiological 

 and (2) psychical forces. Prominent among the former, as causes 

 of modification, are worthy of mention those connected with gen- 

 esis itself: as heredity, physiological selection, sexual selection, 

 hybridity, primogenital selection, and what I would call sexual 

 differentiation, and j:)hiloprogeneity. 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 Dar- 

 win 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 phys- 

 ical 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 expla- 

 nation 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 environ- 

 ment, we include all influences on organisms which act from with- 

 out, and in carefully considering them we shall find it difficult to 

 draw the line between those which are really external and inde- 

 pendent of any motive or inherent tendency in the organism, and 

 those which are not. Hence, the general term " external con- 

 ditions" is resolvable into various minor factors. Considering 

 the inffuences 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 nat- 

 ural 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 expres- 

 sion of one form of manifestation of force, it has as much a classificatory value as physical 

 or psychical. 



