)6 The Destiny of Man. 



like the earth, and a planet like our moon ? 

 Yet these things are simply examples of 

 cosmical matter at four different stages of 

 cooling. The physical differences between 

 steam, water, and ice afford a more famil- 

 iar example. In the organic world the per- 

 petual modification of structures that has 

 been effected through natural selection ex- 

 hibits countless instances of differences in 

 kind which have risen from the accumu- 

 lation of differences in degree. No one 

 would hesitate to call a horse's hoof differ- 

 ent in kind from a cat's paw ; yet a hoof 

 is made up of five claws grown together 

 and furnished with a nail in common. The 

 most signal differences in kind are wont to 

 arise when organs originally developed for 

 a certain purpose come to be applied to a 

 very different purpose, as that change of 

 the fish's air-bladder into a lung which ac- 

 companied the first development of land 

 vertebrates. But still greater becomes the 

 revolution when a certain process goes on 

 until it sets going a number of other proc- 



