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, (6) 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 
