CHAPTER VIII. 



EVIDENCES OF THE THEORY OF NATURAL 

 SELECTION. 



I WILL now proceed to state the main arguments in 

 favour of the theory of natural selection, and then, in 

 the following chapter, the main objections which have 

 been urged against it. 



In my opinion, the main arguments in favour of the 

 theory are three in number. 



First, it is a matter of observation that the struggle 

 for existence in nature does lead to the extermination 

 of forms less fitted for the struggle, and thus makes 

 room for forms more fitted. This general fact may be 

 best observed in cases where an exotic species proves 

 itself better fitted to inhabit a new country than is some 

 endemic species which it exterminates. In Great 

 Britain, for example, the so-called common rat is a 

 comparatively recent importation from Norway, and 

 it has so completely supplanted the original British rat, 

 that it is now extremely difficult to procure a single 

 specimen of the latter : the native black rat has been 

 all but exterminated by the foreign brown rat. The 

 same thing is constantly found in the case of imported 

 species of plants. I have seen the river at Cambridge 

 so choked with the inordinate propagation of a species 



