344 EVOLUTION, OLD AND NEW. 



the survival of the fittest ly means of minute fortuitous 

 indefinite variations. 



"Among many other obligations which the author 

 has to acknowledge to Professor Huxley, are the point- 

 ing out of this very difficulty, and the calling his at- 

 tention to the striking resemblance between certain 

 teeth of the dog, and of the thylacine, as one instance, 

 and certain ornithic peculiarities of pterodactyles as 

 another." * 



In brief then, changed distribution of use and disuse 

 in consequence of changed conditions of the environ- 

 ment is with Lamarck the main cause of modification. 

 According to Mr. Darwin natural selection, or the sur- 

 vival of favourable but accidental variations, is the 

 most important means of modification. In a word, 

 with Lamarck the variations are definite; with Mr. 

 Darwin indefinite. 



* ' Genesis of Species,' p. 76, ed. 1871. 



