THE FACTOKS OF ORGANIC EVOLUTION. 47 



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 unlikeness 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 common, 

 which are consequent on the action of the medium in 

 which they exist : using the word medium in a compre 

 hensive sense, as including all physical forces falling upon 

 them as well as matters bathing them. And we may con 

 clude 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 matters 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 immensely greater than the 

 extent to which it modifies inorganic bodies. 

 i/ 



Here, not discriminating among the special effects which 



