THE ARGUMENTS FROM DISTRIBUTION. 391 



races of organisms, ever being thrust by pressure of popula- 

 tion into new habitats, undergo modifications of structure as 

 they diverge more and more widely in space, it follows that, 

 speaking generally, the widest divergences in Space will 

 indicate the longest periods during which the descendants 

 from a common stock have been subject to modifying con- 

 ditions ; and hence that, among organisms of the same 

 group, the smaller constrasts of structure will be limited 

 to the smaller areas. This we find: "varieties being/ 

 as Dr Hooker says in his Flora of Tasmania, " more re- 

 stricted in locality than species, and these again than 

 genera." Again, if races of organisms spread, and 



as they spread are altered by changing incident forces ; it 

 follows that where the incident forces vary greatly within 

 given areas, the alterations will be more numerous than in 

 equal areas which are less- variously conditioned. This, too, 

 proves to be the fact. Dr Hooker points out that the most 

 uniform regions have the fewest species ; while in the most 

 multiform regions the species are the most numerous. 



§ 139. Let us consider next, how the hypothesis of 

 evolution corresponds with the facts of distribution, not over 

 "different areas, but through different media. If all forms of 

 organisms have descended from some primordial simplest 

 form, it follows that, since this primordial simplest form 

 must have inhabited some one medium out of the several 

 media which organisms now inhabit, the peopling of other 

 media by its descendants, implies migration from one 

 medium to others — implies adaptations to media quite imlike 

 the original medium. To speak specifically — water being 

 the medium in which the lowest living forms exist, it is 

 implied that the earth and the air have been colonized from 

 the water. Great difficulties appear to stand in the way of 

 this assumption. Ridiculing those who contend for the uni- 

 serial development of organic forms, who have, indeed, laid 

 themselves open to ridicule by their many untenable pro- 



