CAUSES OF GENETIC VARIATION 229 



the variation previously observed in the species was afterwards 

 shown to be due to fungoid disease. The circumstances to which 

 he mainly points in support of his view is that the mutation bred 

 true, but this is only evidence of its genetic distinctness, which 

 may, of course, be admitted by those who remain unconvinced 

 as to the original cause of its appearance. He adds that he is 

 making similar experiments with some twenty genera; but what 

 is more urgently needed is repeated confirmation of the original 

 observation. When it has been shown that this mutation can 

 be produced with any regularity from a plant which does not 

 otherwise produce it on normal self -fertilisation, the enquiry 

 may be profitably extended to other plants. 



A curious and novel experiment, which however, led ulti- 

 mately to a negative result, was made by F. Payne. Many dis- 

 cussions have been held respecting the blindness of cave animals. 

 The phenomenon is one of the well-known difficulties, and most 

 of us would admit that the theory of evolution by the natural 

 selection of small differences does not offer a really satisfying ac- 

 count of it. Those who believe in the causation of such modifica- 

 tions by environmental influences and in their hereditary trans- 

 mission make, of course, the simple suggestion that the darkness 

 is the cause of the loss of sight, and that disuse has led to the 

 reduction of the visual organs. Payne bred Drosophila ampelo- 

 phila, the pomace-fly (which is easy to keep in confinement, fed 

 on fermenting bananas) , for sixty-nine generations in darkness. 

 At the end of that period there was no perceptible change in the 

 structure of the eyes, or in any other respect. The number of 

 generations may possibly be regarded as insufficient to prove 

 anything, but comparing them, as he does, with the generations 

 of mankind, we see that they correspond with a period of about 

 two thousand years, an interval far longer than those which 

 many writers in particular cases have deemed sufficient. 



In his first paper Payne states that, though no structural 

 difference could be perceived, the flies which had been bred in 

 the dark reacted less readily to light than those which had been 

 reared under normal conditions, and he inclined to think that 

 the treatment had thus produced a definite effect. After more 



