212 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



which in Neritina is very small, is modified in Navicella into 

 a large thin plate appearing as a direct prolongation of the 

 free portion of the operculum ; finally this lies in a small fold 

 between the back of the foot and the horizontal deck inside the 

 back of the shell. Thus it is evident that the operculum of 

 Navicella comes under the head of rudimentary organs, for it 

 can neither revolve through an arc of 180, as it would have to 

 do to close the shell, nor could it close the aperture even if it 

 were brought into the proper position. Now, as Navicella is in 

 every other respect a true Neritina, it may be regarded as one 

 which, by long inurement to living in rushing mountain streams, 

 has had its shell modified in the way most suited to those con- 

 ditions, while the operculum, in consequence of long disuse, has 

 become a peculiar degenerate or rudimentary organ. 100 



The foregoing cases show us that sometimes even the 

 weakest currents may act as potent means of selection between 

 different species or individuals, and they also prove that animals 

 nearly allied, or individuals of the same species, may be quali- 

 fied at different stages of growth to resist the strongest cur- 

 rents or the most violent shocks by some modification in their 

 mode of life. The characters thus developed clearly exhibit 

 their connection with the creature's mode of life, since its 

 power of resistance evidently depends on them ; and in the 

 instances where, as in Navicella, rudimentary organs occur, 

 their derivation from organs formerly of physiological value 

 may be attributed to the indirect action of the currents. But 

 it must not, at the same time, be forgotten that the degene- 

 ration of an organ is, in point of fact, no more explained by 

 its disuse than the continued existence of a still serviceable 

 organ, as e.g. the eye, is explained by the fact of its utility or 

 by the evidence of the vast importance of its use. 



Nevertheless, there are a few instances in which the direct 

 effects of more or less constant currents of different force on the 

 form of certain animals admit of easy proof. 



The direct mechanical effect of currents on the structure 

 and growth of animals. It is evident that this influence 

 cannot be exerted in any considerable degree on any but fixed 

 animals, at any rate in such a way as to modify or determine 



