66 CHARLES DARWIN. 



at once tend to increase. " Nature may be compared 

 to a surface on \fchich rest ten thousand sharp wedges 

 touching each other and driven inward by incessant 

 blows." Darwin meant by this image to express 

 that just as any single wedge would instantly rise 

 above the rest when the blows on it were in any 

 way lessened as compared with those on the other 

 wedges, so it would be with the proportionate number 

 of any species when the checks to which it is sub- 

 jected are in any way relaxed. 



If the external conditions alter, and the changes 

 continue progressing, the inhabitants will be less well 

 adapted than formerly. The changed conditions 

 would act on the reproductive system and render the 

 organisation plastic. Now, can it be doubted, from 

 the struggle each individual has to obtain subsistence, 

 that any minute variation in structure, habits, or 

 instincts adapting that individual better to the new 

 conditions would tell upon its vigour and health ? 

 " Yearly more are bred than can survive ; the smallest 

 grain in the balance, in the long run, must tell on 

 which death shall fall, and which shall survive." If 

 this went on for a thousand generations who will deny 

 its effect " when we remember what, in a few years, 

 Bakewell effected in cattle, and Western in sheep, by 

 this identical principle of selection " ? 



He gives an imaginary example of a canine animal 

 preying on rabbits and hares. If the rabbits, con- 

 stituting its chief food, gradually became rarer, 

 and the hares more plentiful, the animal would be 



