CAVE FISHES 389 



Just how the eyes of any blind fish species were led to disappear, we 

 cannot say. An old idea was that where the eye had become useless, there 

 was a positive incentive for eliminating the organ, since this would save 

 energy both in adulthood and — especially — during growth. This notion 

 seems ridiculous nowadays, for the proportion of a growing animal's 

 food-intake which goes to enlarge the eye is negligible. Most of the 

 energy released from food goes for motor and secretory activity, and 

 only a very small part of the food is converted into new protoplasm. 

 Nor does the disappearance of an eye leave a hole in the head — its 

 volume is occupied by tissues (mainly muscle) which consume just as 

 much energy as the eye had done. 



Though a normal eye is excess baggage to a cavernicolous or limico- 

 lous fish, there appears to be no urgent reason why he should get rid of 

 it. Useless organs do not always promptly disappear simply because they 

 have become useless — as witness the human appendix, coccyx, platysma, 

 tonsils, wisdom teeth, et al. We are left to suppose that in the immediate 

 outside ancestors of most cave species the eye was 'trying' to disappear 

 anyway, but was prevented from doing so, by natural selection, because 

 it was useful and necessary. The usefulness once removed by the assump- 

 tion of cavernicolous life, the inherent tendency for the eye to shrink was 

 allowed to express itself, even unto the logical end-result — complete loss. 



This explanation does not tax the imagination of ichthyologists as 

 severely as one might think. In many an open-water fish species, reduced- 

 eyed individuals appear as soon as the food supply is made abundant 

 and predatory enemies are removed. Lack of competition then permits 

 the full development of individuals which, since their germ-plasm has 

 undergone 'mutations of loss', would formerly have been suppressed by 

 starvation or capture. Loss-mutations are known particularly to affect 

 the more complex organs of vertebrates, such as the eye. A species or 

 family in which such mutations occur with especial frequency has of 

 course no advantage, over others, in any attempt to become adjusted to 

 a habitat in which the illumination is reduced or absent. But if a group 

 which throws loss-mutations also produces an imusual number of other 

 trial-and-error modifications (as seems likely) , then such a group might 

 readily evolve the dermal sense-organs, barbels, or whatnot required to 

 cope with a dim-light environment. Once adapted to dim-light existence, 

 such a group would actually be better off in a cave, if it happened to 

 find one, than outside where there were predators to be dodged. And 

 once inside the cave for good, a rapidly-mutating species would inevit- 



