VARIA TION NOT CONCOMITANT. 17 



BLIND CAVE-CRABS. 



The cave-crabs which have lost their disused 

 eyes but not the disused eye-stalks appear to illus- 

 trate the effects of natural selection rather than 

 of disuse. The loss of the exposed, sensitive, and 

 worse-than-useless eye, would be a decided gain, 

 while the disused eye-stalk, being no particular 

 detriment to the crab, would be but slightly 

 affected by natural selection, though open to the 

 cumulative effects of disuse. The disused but 

 better protected eyes of the blind cave-rat are 

 still " of large size " (Origin of Species ; p. no). 



NO CONCOMITANT VARIATION FROM 

 CONCOMITANT DISUSE. 



It is but fair to add that these instances of the 

 cave-crab's eye-stalk and the closely-packed teeth 

 are put forward by Mr. Spencer with the more 



C 



