THE ARGUMENTS FROM DISTRIBUTION. 4^9 



species "most of the local varieties are connected with their 

 parent form by individuals exhibiting all the shades of 

 variation." 



Further general relationships are to be inferred. If 

 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 contrasts 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 rela- 

 tively 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 evo- 

 lution corresponds with the facts of distribution, not over 

 different areas but through different media. If all forms of 

 ~»rganisms have descended from some primordial form, it 

 oilows that since this primordial form must have inhabited 

 some one medium out of the several media now inhabited, 

 the peopling of other media by its descendants implies 

 migration from one medium to others — implies adaptations 

 to media quite unlike the original medium. To speak speci- 

 fically — water being the medium in which the lowest living 

 form's exist, the implication is that the earth and the air 

 have been colonized from the water. Great difficulties 



