THE FACTOES OP ORGANIC EVOLUTION. 49 



much by rain and wind as its interior gains by further 

 oxidation of the iron. Most mineral masses pebbles, 

 boulders, rocks if they show any effect of the environment 

 at all, show it only by that disintegration of surface 

 which follows .the freezing of absorbed water : an effect 

 which, though mechanical rather than chemical, equally 

 illustrates the general truth. Occasionally a &quot; rocking- 

 stone &quot; is thus produced. There are formed successive 

 layers relatively friable in texture, each of which, thickest 

 at the most exposed parts, and being presently lost by 

 weathering, leaves the contained mass in a shape more 

 rounded than before; until, resting on its convex under- 

 surface, it is easily moved. But of all instances perhaps 

 the most remarkable is one to be seen on the west bank of 

 the Nile at Philae, where a ridge of granite 100 feet high, 

 has had its outer parts reduced in course of time to a 

 collection of boulder-shaped masses, varying from say a 

 yard in diameter to six or eight feet, each one of which 

 shows in progress an exfoliation of successively-formed 

 shells of decomposed granite : most of the masses having 

 portions of such shells partially detached. 



If, now, inorganic masses, relatively so stable in com 

 position, thus have their outer parts differentiated from 

 their inner parts, what must we say of organic masses, 

 f characterized by such extreme chemical instability ? 

 instability so great that their essential material is named 

 protein, to indicate the readiness with which it passes 

 from one isomeric form to another. Clearly the necessary 

 inference is that this effect of the medium must be 

 wrought inevitably and promptly, wherever the relation 

 of outer and inner has become settled : a qualification for 

 which the need will be seen hereafter. 



Beginning with the earliest and most minute kinds 

 of living things, we necessarily encounter difficulties in 

 getting direct evidence; since, of the countless species 



