THE ARGUMENTS FROM DISTRIBUTION. 477 



recently-published instances of the usurpations of areas, and 

 changes of distribution hence resulting. In the Natural His- 

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

 from some New Zealand naturalists : " You would be sur- 

 prised 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 (Rumex obtusifolius or R. 

 crispus) 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, 

 * To avoid circumlocution I let these words stand, though they are not 

 truly descriptive; for the prosperity of imported species is largely, if not 

 mainly, caused by the absence of those natural enemies which kept them down 

 at home. 



