170 THE DARWINIAN THEORY 



the early stages of development of the higher Verte- 

 brates is very interesting, and the only possible 

 explanation of their presence is that their possessors 

 are descended from gill-breathing ancestors. These 

 gill-slits never bear gills, and all close up early with 

 the exception of one, the hyo-mandibular cleft, which 

 is preserved because its function is changed. This 

 cleft passes close to the ear, which is buried in the 

 side of the skull, and by remaining open becomes 

 advantageous for the purpose of hearing. So what 

 was once a gill-cleft has by change of function 

 become part, of the organ of hearing. 



The electric organs of some fish are good ex- 

 amples of change of function. These are always 

 formed by modification of muscular tissue, and in 

 all muscular contractions electric changes occur. 

 Perhaps this is an instance of a secondary function 

 becoming primary. In Gymnotus, the electric eel, 

 the electric organs lie just underneath the skin along 

 the sides of the tail. In Malapterurus, the electric 

 cat-fish, they extend over the whole body between 

 the skin and muscles, being especially developed at 

 the sides. In Torpedo they are used for stunning the 

 animals on which it preys, and also for purposes of 

 defence. 



The Insufficiency of Time. 



Natural Selection is a slow process depending on 

 the gradual accumulation of small variations, for the 

 acquirement of which we have no actual standard of 

 time. Palaeontology as yet tells us nothing as to the 

 origin of life, or even the origin of the large groups 



