MAN AND APES. 185 



most careful observations having as yet en- 

 abled us to discover what conditions determine 

 in these exceptional cases such a marvellous 

 metamorphosis. 



It is true that the Axolotl has characters 

 of immaturity, and that the form ultimately 

 attained by it is probably the fully-developed 

 condition ; but the wonder is thus only in- 

 creased, since while the ordinary and imma- 

 ture Axolotls breed freely, the rarely-developed 

 adults are absolutely sterile. 



To revert from this digression, however, to 



the question of the cause and mode of specific 



origin. I have elsewhere* endeavoured to 



show, by many different facts, what is the 



teaching of nature as to such origin — namely, 



that very frequently indeed similarity of 



structure may arise without there being any 



genetic affinity between the resembling forms,t 



as also that it is much rather to an internal 



cause or principle than to any action of 



* ■ Genesis of Species,' 2nd edition, 1872. 

 t ' Genesis of Species,' p. 71, chap, iii., on the co- 

 existence of closely similar structures of diverse origin. 



