HERITABLE DIFFERENCES 53 



Reverse mutations have also been repeatedly ob- 

 served. This is something resembling the unscrambling 

 of an egg. Morgan and Bridges obtained, for exam- 

 ple, normal red-eyed flies from white-eyed mutants and 

 May, also with ubiquitous Drosophila, got back nor- 

 mal-eyed individuals from bar-eyed mutants. 



The frequent occurrence of recurrent mutations, 

 that is, the reappearance of the same mutations, sug- 

 gests that the cause underlying these irregular heredi- 

 tary changes is something continuous and definite even 

 if we are at present unable always to put our finger 

 upon it. The evening primroses have repeatedly shown 

 the same mutations in widely different localities and 

 under the eyes of different investigators. Morgan says 

 of his famous banana flies, "One of the first mutants 

 that appeared, viz., white eyes, has appeared anew in 

 our cultures about three times, in cultures known to 

 be free from it before and not contaminated. The same 

 mutant has been found by several other observers. 

 The eye color vermilion has appeared at least six 

 times; the wing character called rudimentary, five 

 times ; cut wing has been found four times," etc. 



7. THE ORIGIN OF MUTATIONS 



Mutations may be gametic, zygotic or somatic in 

 their origin. There seems to be no reason why muta- 

 tion may not occur at any stage in the life-cycle of an 

 organism. In the first place, it may be gametic in 

 origin if the onset is in the germ-cell before or during 

 the maturation changes that prepare it for union with 



