PROBLEM OF CAVE BLINDNESS 251 



should serve also for the gander. Some cave animals 

 have normal eyes, and a few animals that live in the 

 open, like the shore-fish Typhlogobius, are blind. 



What is the other theory? Simply that the 

 blindness arose as a germinal variation or muta- 

 tion, and that it suited cave-life. For a useless 

 organ is a weak spot. On this view there is no 

 difficulty in the rarity of rudimentary eyes in open- 

 air conditions, for variations in an obviously dis- 

 advantageous direction tend to be eliminated. The 

 difficulty is rather in finding facts to justify the 

 belief that the occurrence of variations in the 

 direction of blindness may be postulated with some 

 show of reasonableness. It is here that the recent 

 work of Loeb comes in. This brilliant experimenter 

 of the Rockefeller Institute has found that it is 

 quite easy to produce a percentage of fish-embryos 

 (Fundulus) with defective eyes (a) by unsuitable 

 crossing, (b) by adding a little potassium cyanide 

 to the water, or (c) by exposing the developing eggs 

 to very low temperatures. It is not suggested that 

 the ancestors of the blind cave-animals became 

 blind as the result of parental mismating, or of 

 poisoning, or of great cold; what the experiments 

 show is that relatively slight external changes may 

 so alter the constitution of the germ that a leap is 

 taken in the direction of blindness. Therefore in 

 our theory of the origin of cave-blindness it is quite 

 legitimate to start with postulating heritable 

 germinal mutations which we cannot definitely 

 account for. On this view, the tendency towards 



