THE FACTORS OF ORGANIC EVOLUTION. S9 



agencies.* If a difference between the quantities of a force which 

 acts on two organisms, otherwise alike and otherwise similarly condi- 

 tioned, produces some difference between them ; then, by implication, 

 this force produces in both of them effects which they show in common. 

 The inequality between two things cannot have a value unless the 

 things themselves have values. Similarly if, in two cases, some unlike- 

 ness of proportion among the surrounding inorganic agencies to which 

 two plants or two animals are exposed, is followed by some unlikeness 

 in the changes wrought on them ; then it follows that these several 

 agencies taken separately, work changes in both of them. Hence we 

 must infer that organisms have certain structural characters in com- 

 mon, which are consequent on the action of the medium in which they 

 exist : using the word medium in a comprehensive sense, as including 

 all physical forces falling upon them as well as matters bathing them. 

 And we may conclude that from the primary characters thus produced 

 there must result secondary characters. 



Before going on to observe those general traits of organisms due 

 to the general action of the inorganic environment upon them, I feel 

 tempted to enlarge on the effects produced by each of the several mat- 

 ters and forces constituting the environment. I should like to do this 

 not only to give a clear preliminary conception of the ways in which 

 all organisms are affected by these universally-present agents, but also 

 to show that, in the first place, these agents modify inorganic bodies 

 as well as organic bodies, and that, in the second place, the organic 

 are far more modifiable by them than the inorganic. But to avoid undue 

 suspension of the argument, I content myself with saying that when 

 the respective effects of gravitation, heat, light, &c, are studied, as 

 well as the respective effects, physical and chemical, of the matters 

 forming the media, water and air, it will be found that while more or 

 less operative on all bodies, each modifies organic bodies to an extent im- 

 mensely greater than the extent to which it modifies inorganic bodies. 



Here, not discriminating among the special effects which these vari- 

 ous forces and matters in the environment produce on both classes of 

 bodies, let us consider their combined effects, and ask What is the 

 most general trait of such effects ? 



* It is true that while not deliberately admitted by Mr. Darwin, these effects are not 

 denied by him. In his Animals and Plants under Domestication (vol. ii, 281), he refers 

 to certain chapters in the Principles of Biology, in which I have discussed this general 

 inter-action of the medium and the organism, and ascribed certain most general traits to 

 it. But though, by his expressions, he implies a sympathetic attention to the argument, 

 he does not in such way adopt the conclusion as to assign to this factor any share in the 

 genesis of organic structures much less that large share which I believe it has had. I 

 did not myself at that time, nor indeed until quite recently, see how extensive and pro- 

 found have been the influences on organization which, as we shall presently see, are 

 traceable to the early results of this fundamental relation between organism and medium. 

 I may add that it is in an essay on " Transcendental Physiology," first published in 1857, 

 that the line of thought here followed out in it wider bearings, was first entered upon. 



