VII INDEPENDENT ORIGIN OF SENSE ORGANS 337 



(auditory and visual) often have a quite similar structure in 

 animals by no means closely related, and this in cases where 

 they must have arisen independently, because the common 

 ancestors of both, and even the ancestral forms of both 

 derived from the latter, were entirely without such organs. 



Certain Medusae and worms, for example, have perfectly 

 similar auditory organs, although these cannot have been 

 derived one from the other — indeed the auditory organs 

 of many Medusae and worms are more closely similar to one 

 another than to those of other Medusae or other worms. 

 Consider the eyes of vertebrates and cuttle-fishes. In the 

 latter is repeated the whole plan of structure of the retina of 

 the former, only in the reverse order : in the vertebrate eye the 

 expansion of the optic nerve lies on the inside of the retina : 

 in the invertebrate on the outside. In the slug Onchidium. 

 however, as Semper has shown, we find the same relations as 

 in the vertebrate. All these three kinds of eyes must then 

 have arisen independently, and yet they are constructed of 

 the same parts. 



" This fact," I insisted further, " is of quite peculiar im- 

 portance. It shows in the clearest way . .. . that in conse- 

 quence of the relations of organisms to particular influences 

 of the external world perfectly similar forms may arise, even 

 without any immediate blood-relationship between them, not, 

 on my view, because the material provided in the animal 

 organism had little capacity for evolution, but because with 

 this material organs can only be constructed within narrow 

 limits of variation to fulfil in the best j)ossible degree a 

 perfectly definite and constant requirement from without." 



I said further that the anatomy of the sense organs offers 

 specially numerous examples of this in two directions ; that 

 notwithstanding the absence of all direct relationship of 

 descent (1) resemblance, or exact similarity of form, has 

 arisen ; (2) that similar forms, but in different combination, 



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