248 PHYSICAL BASIS OF HEREDITY 



great variety of types shown by nearly all of our domesti- 

 cated animals and plants — varieties that follow MendePs 

 law — appears to give further support to the view that the 

 process of mutation is widespread. 



One of the most interesting phenomena connected with 

 mutation is the recurrence of the same change. It has 

 long been recognized that certain *^ sports'' such as albi- 

 nos and melanic forms are found again and again in 

 nature. In insects there are many records of the sporadic 

 appearance of the same type, such as the light form (lacti- 

 color) of the moth Abraxas. It is true that not all such 

 appearances are to be accepted offhand as the first appear- 

 ance of the mutative change, since when these are reces- 

 sive it is probable in most cases * that the actual mutation 

 occurred several generations before the mutated genes 

 came together to produce the mutant character. But 

 granting this, it is at least probable that the same type 

 has appeared in many cases independently. The only 

 evidence that can be relied upon in such cases is from 

 pedigreed cultures, followed up by evidence that the 

 mutants that look alike are really due to mutations in the 

 same locus. Fortunately there is actual evidence, both 

 for plants and for animals, that can be appealed to to show 

 that the same mutations recur. 



The most extensive evidence is from Drosophila 

 melanogaster. 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; truncate wing has frequently appeared, but has 

 not necessarily been always produced by the same change. 

 Certain characters such as notch wings, that have 



* Recessive mutations in the X-chromosomes of the XX-XY type may 

 appear in the male in the next generation. 



