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.”* 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. 
