84 DEGENERATION [CHAP. 



members are used they have been converted 

 into paddle-like structures. Four of the horse's 

 toes have actually gone, but the " splint bones " 

 which supported them remain as rudiments. 

 In the cow the hindermost toes still remain 

 as useless appendages. Disuse has brought 

 about such degenerations contemporaneously 

 with developments in other directions ; just as 

 a tadpole required a tail for swimming, but this 

 organ becomes absorbed when it develops into 

 a terrestrial frog. As natural selection has 

 obviously nothing to do with this disappear- 

 ance of the tail of a tadpole, why should it be 

 required in all the cases of toes ? 



In plants there are certain great causes of 

 degeneration, e.g., the want of, and an excess 

 of, water, as well as great cold and great heat, 

 and diminished light. For a plant to grow 

 luxuriantly, it requires an optimum of each of 

 these conditions for its various functions, 

 respectively. Rarely, if ever, do plants secure 

 all the optima together in any one place of the 

 globe. Perhaps the nearest approach is seen 

 in a moist tropical climate, to which the flora 

 has become adapted. At all events, we find it 

 is there that the most luxurious vegetation 

 flourishes. Taking such as a standard, plants 

 not only decrease in size until they are tiny 

 dwarfs, as willows in Arctic regions, but their 

 various organs, as leaves, become, not only 



