98 ■ ORGANIC EVOLUTION— THE FACTORS 



whom, since they have migrated to dark places, sight 

 is useless ? I think there is abundant evidence proving 

 that it has so aided. An eye, if useless, must be much 

 worse than Useless to the animal possessing it. In its 

 exposed position it is in constant danger of injury, and, 

 from the delicate nature of the organ, harm to it more 

 disastrously affects the whole organism than harm to 

 an equal extent of any other part of the surface. Eye- 

 less animals, or animals with small rudimentary eyes, 

 therefore, are at an advantage in the dark, as compared 

 to animals with fully-developed eyes. The latter would 

 therefore tend to be eliminated, while the former would 

 tend to survive and have offspring. Accordingly we 

 find that certain cavern-inhabiting Crustacea, descended 

 from stalk-eyed ancestors, have lost their eyes, but not the 

 stalks on which the eyes were carried. The loss of the 

 eyes cannot be set down to Cessation of Selection alone, 

 for, since the stalks must have appeared later in the 

 phylogeny than the eyes, the former are presumably 

 the less stable structures, and therefore liable to dis- 

 appear sooner on Cessation of Selection, whereas they 

 have persisted. In like manner the disappearance of 

 the eyes cannot be attributed entirely to Cessation of 

 Use, for, if Cessation of Use is a cause of specific 

 degeneration, it ought to have caused an equal or 

 indeed greater retrogression of the less stable stalks. 

 Therefore in this case we can account for the persistence 

 of the stalk, when the eye has disappeared, only by 

 supposing that Natural Selection — reversed Natural 

 Selection — lias co-operated with Cessation of Selection 

 (or Cessation of Use, according to the theory we hold) 

 to remove the worse than useless eye, leaving the. no 

 more useful but much less harmful stalk persistent. 

 Similarly we can explain the fact that the degenerate 

 eyes of the proteus are covered with skin neither by 

 the theory of Cessation of Selection nor by the theory 



