

48 THE FACTORS OF ORGANIC EVOLUTION. 



these various 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 ? 



Obviously the most general trait is the greater amount 

 of change wrought on the outer surface than on the inner 

 mass. In so far as the matters of which the medium is 

 composed come into play, the unavoidable implication is 

 that they act more on the parts directly exposed to them 

 than on the parts sheltered from them. And in so far as 

 the forces pervading the medium come into play, it is 

 manifest that, excluding gravity, which affects outer and 

 inner parts indiscriminately, the outer parts have to bear 

 larger shares of their actions. If it is a question of heat, 

 then the exterior must lose it or gain it faster than the 

 interior; and in a medium which is now warmer and now 

 colder, the two must habitually differ in temperature to 

 some extent at least where the size is considerable. If 

 it is a question of light, then in all but absolutely trans 

 parent masses, the outer parts must undergo more of any 

 change producible by it than the inner parts supposing 

 other things equal ; by which I mean, supposing the case 

 is not complicated by any such convexities of the outer 

 surface as produce internal concentrations of rays. Hence 

 then, speaking generally, the necessity is that the primary 

 and almost universal effect of the converse between the 

 body and its medium, is to differentiate its outside from its 

 inside. I say almost universal, because where the body is 

 both mechanically and chemically stable, like, for instance, 

 a quartz crystal, the medium may fail to work either inner 

 or outer change. 



Of illustrations among inorganic bodies, a convenient 

 one is supplied by an old cannon-ball that has been long 

 lying exposed. A coating of rust, formed of flakes within 

 flakes, incloses it ; and this thickens year by year, until, 

 perhaps, it reaches a stage at which its exterior loses as 



