THE PHENOMENA OF MUTATION 189 



describes a mutation in Drosophila consisting in 

 the loss of the eyes, and triumphantly remarks : 

 ' Formerly we were taught that eyeless animals 

 arose in caves. This case shows that they may also 

 arise suddenly in glass milk-bottles by a change in 

 a single factor.' As it stands the statement is per- 

 fectly true, but it is obvious that the writer does not 

 believe that the darkness of caves ever had anything 

 to do with the loss of eyes. It is almost as though 

 a man should discover that blindness in a certain 

 case was due to a congenital, i.e. gametic, defect, 

 and should then scoff at the idea that any person 

 could become blind by disease. Some of those 

 who specialise in the investigation of genetics seem 

 to give inadequate consideration to other branches 

 of biology. It is a well-established fact that in the 

 mole, in Proteus, and in Amblyopsis (the blind fish 

 of the Kentucky caves), the eyes develop in the 

 embryo up to a certain stage in a perfectly normal 

 way and degenerate afterwards, and that they are 

 much better developed in the very young animal 

 than in the adult. Does this metamorphosis take 

 place in the blind Drosophila of the milk-bottle ? 

 The larva of the fly is, I believe, eyeless like the 

 larvae of other Diptera, but Morgan says nothing of 

 the eye being developed in the imago or pupa and 

 then degenerating. There is therefore no relation or 

 connexion between the mutation he describes and 

 the evolution of blindness in cave animals. It is a 

 truth, too often insufficiently appreciated by biologists, 

 that sound reasoning is quite as important in science 

 as fact or experiment. Loeb * also endeavours to 

 prove that the blindness of cave animals is no 



1 The Organism as a Whole, p. 319 (New York and London, 1916). 



