20 EVOLUTIONS OF ORGANIZATION. 



organ of vision, on the principle that if so complex 

 a structure could be disposed of, no difficulty need 

 be made about any other. Starting from a nerve- 

 extremity coated with pigment as the simplest in 

 " the great kingdom of the Articulata " he sees no 

 difficulty "in believing that natural selection has 

 converted the simple apparatus of an optic nerve 

 merely coated with pigment and invested by trans- 

 parent membrane into an optical instrument, as 

 perfect as is possessed by any member of the great 

 Articulate class." 1 Then, without attempting to 

 explain the vertebrate eye, which, in the individual, 

 is developed like many other structures, through a 

 series of forms utterly incapable of function, he tells 

 the reader that, if he finds the other facts in the 

 book bear out the theory, he is bound to admit that 

 the eye of an eagle might be formed by natural 

 selection. No doubt, the evolution of the vertebrate 

 from the invertebrate eye is made easier by Kowa- 

 levsky's discoveries in the larval ascidians ; but 

 what is called an eye in amphioxus is only a spot 

 without mechanism for vision, so that it is really 

 true that the invertebrate eye is blotted out be- 

 fore the true vertebrate eye, which, in the in- 

 dividual, passes through so many visually useless 

 stages of development, suddenly appears before 



1 Op. cit. pp. 206 and 207. 



