TRANSMISSION OF MODIFICATIONS 



411 



such event there ensues a third stage of degeneracy. Obviously 

 the full effects of selection will depend very much upon the 

 character of the part and its connection with the vital interests 

 of the species. 



The influences just mentioned will sufficiently account for ex- 

 treme degeneracy of a once active part, but they will not account 

 for absolute disappearance. Any character under such conditions 

 would decrease to a low minimum where its presence becomes 

 insignificant, and there it would remain, but it would not abso- 

 lutely disappear except through some other agency. That 

 characters, even those which were once important, do entirely 

 disappear, leaving not even rudimentary parts, is evidenced by 

 the disappearance of the legs of snakes, but that the later stages 

 are extremely slow is shown by the rudimentary leg bones of 

 the python and the whale. 



Degeneration of eyes in cave-dwelling and deep-sea species. 

 It is a well-known fact that the eyes of cave fishes and insects 

 often exhibit all grades of degeneration, from near the normal 

 down to the merest rudiments. Being entirely useless under 

 the conditions of life, selection is suspended, and Nature is 

 having her way with the remnants of what was once a highly 

 developed organ. 



Deep-sea fishes are either in the same condition or else are 

 supplied with enormous eyes of a kind evidently fitted to per- 

 ceive light rather than to make distinct and clear-cut images. 



The gradual failure of parts like the toes that have gone from 

 the horse, or the apparently disappearing vermiform appendix, 

 might be attributed to some failure or imperfection in the germ ; 

 but the instance of disappearing limbs is too clearly connected 

 with disuse, and that of disappearing eyes with lack of what 

 may be called essential conditions of development, to be explained 

 wholly on the ground of imperfection from within as the funda- 

 mental cause of degeneracy. 1 



The final disappearance of a useless part is certainly due to 

 some fact other than the withdrawal or even the reversal of 

 selection. There must be morphological units of some kind 



1 Light is evidently essential to the origin of the sensitive spot we call the eye, 

 especially in the formation of pigment. 



