450 



The Journal of Heredity 



the skin, but it apparently needs little 

 oxygen, as one can keep a large number 

 of them in a pail of water for some time 

 without changing the water and they 

 do not appear to suffer. "When placed 

 in an aquarium where there is a division 

 into two parts, one light and one dark, 

 the fish will always seek the dark. The 

 optic lobes of the brain have degenerated 

 greatly, but in other respects the brain 

 shows little change. There are many 

 other adaptions, but a sufficient number 

 have been mentioned to indicate the 

 changes which have taken place in this 

 remarkable fish. 



Fig. 4 also shows a member of the 

 genus Fundulus, the killifishes, or 

 minnows. There are more than a 

 score of species in this genus, and the 

 various groups include many different 

 types, but this one is possibly distantly 

 related to Amblyopsis and may give an 

 indication of how the ancestor of 

 Amblyopsis looked, and how the genus 

 Amblyopsis might look today, if adap- 

 tive selection had not allowed the 

 deformed members to establish the 

 status of the species rather than the 

 normal members. Such an evolution 

 can be paralleled in some himian socie- 

 ties where misfits are perpetuated and 

 crowd out the normal population. A 

 degenerate clan like the "Jukes" may 

 thus take possession of a mountain 

 valley and fix the tone of the whole 

 population. 



HOW EYESIGHT WAS LOST 



The question of how the cave fish 

 lost its eyes has long been one of the 

 stock subjects for debate among stu- 

 dents of evolution. Even to review the 

 hypotheses that have been advanced 

 would require a long article. They 

 may, of course, be broadly divided in 

 two groups: on the one side are the 

 Lamarckians who think that the eye 

 atrophied as the result of disuse and 

 that the effects of this disuse were 

 inherited, with cumulative results in 

 each generation; on the other side are 

 the Darwinians who look to some form 

 of natural selection, acting on variation, 

 to produce the obser\'ed result. 



The natural inethod of deciding 

 between the conflicting views would 

 be by experiment, and a number of 

 biologists have conducted experiments 

 with such fish, but the results have in 

 no case been conclusive, probably owing 

 to the short time they covered, as 

 compared with the long time in which 

 the fish ma>' have been undergoing 

 evolution. 



It is quite likely that various species 

 have reached their eyeless condition in 

 various ways, and that even on one 

 species more than one cause has been 

 operating to produce the absence of 

 vision. There are at least two explana- 

 tions which can be plausibly supported 

 by modern knowledge of genetics. 



1. It is possible that the blindness was 

 produced rather suddenly as a result of 

 hybridization, exposure to the action 

 of chemicals, or some other powerful 

 influence. Blindness can be experi- 

 mentally produced in fish by these 

 means. The fish once being blind, 

 they wandered into a cave — or perhaps 

 were in a cave when they became blind— 

 and were able to survive because eyes 

 were not of \'alue in cave-darkness 

 anyway. If they had stayed outside 

 the cave they naturally would have 

 perished. In this view the cave is not 

 responsible for the blindness, either 

 directly or indirectly ; it merely preserves 

 the fish who have become blind for 

 some other reason. This is the explana- 

 tion which has recently been advocated 

 by Jacques Loeb,- among others. 



2. The commoner explanation looks 

 on the blindness as being of slower 

 development, although not, as the 

 Lamarckians supposed, produced by 

 cave darkness. It is supposed that a 

 number of normal fish got into a cave. 

 Those with the best sight would find 

 their way out; those with naturally 

 defective vision would be unable to 

 escape, and, remaining there, wt)uld 

 I)erpetuatc their kind. In the cave 

 darkness acute vision would no longer be 

 essential to survival. There being no 

 predaccous enemies, the fish which 

 coukl not see at all would have just as 

 good a chance of surviving and leaving 



2 The Organism as a Whole, Chapter xii. New York, 1916. 



