THE ARGUMENTS FROM DISTRIBUTION. 389 



changes of distribution hence resulting. In the Natural His- 

 tory Review for January, 1864, Dr Hooker quotes as follows 

 from sorneJNew Zealand naturalists : — " You would be surprised 

 at the rapid spread of European and other foreign plants in 

 this country. All along the sides of the main lines of road 

 through the plains, a Polygonum {aviculare), called ' Cow 

 Grass/ grows most luxuriantly, the roots sometimes two feet 

 in depth, and the plants spreading over an area from four to 

 five feet in diameter. The dock {Bumex obtusifolius or B. 

 crisjjus) is to be found in every river bed, extending into the 

 valleys of the mountain rivers, until these become mere tor- 

 rents. The sow-thistle is spread all over the country, growing 

 luxuriantly nearly up to 6000 feet. The water-cress increases 

 in our still rivers to such an extent, as to threaten to choke 

 them altogether : * * * I have measured stems twelve feet 

 long and three-quarters of an inch in diameter. In some of 

 the mountain districts, where the soil is loose, the white clover 

 is completely displacing the native grasses, forming a close 

 sward. * * * In fact, the young native vegetation appears 

 to shrink from competition with these more vigorous in- 

 truders." " The native (Maori) saying is, ' as the white 

 man's rat has driven away the native rat, so the European 

 fly drives away our own, and the clover kills our fern, so 

 will the Maoris disappear before the white man himself.' " 



Given this universal tendency of the superior to over- 

 run the habitats of the inferior ; let us consider what, on the 

 hypothesis of evolution, will be the effects on the geo- 

 graphical relationships of species. 



§ 138. A race of organisms cannot expand its sphere of 

 existence, without subjecting itself to new external conditions. 

 Those of its members which spread over adjacent areas, 

 inevitably come in contact with circumstances partially 

 different from their previous circumstances ; and such of 

 them as adopt the habits of other organisms, necessarily 

 experience re-actions more or less contrasted with the re- 



