CORRELATION 175 



Drosophila resemble generic or family characters which dis- 

 tinguish other groups. But even in Drosophila there is practi- 

 cally no evidence as to the genetic basis of the characters used 

 to separate species in that genus. The geneticist, naturally 

 enough, has concentrated on the mutations most easily observed 

 and studied. A special search for mutation in characters 

 known to be of specific value seems scarcely to have been 

 attempted. 



We are in great need of information as to whether the unit 

 phenotypic characters are really genotypic units. We may 

 discard for the moment the numerous specific characters 

 which are not unambiguously definable as units and consider 

 only such differences as : number of metameric parts, pre- 

 sence or absence of definite spines or bristles, development of 

 definite coloured patches, etc. These are the sorts of characters 

 which appear in different combinations in allied species and 

 are therefore spoken of as segregating independently. Analogy 

 with the results of genetical studies would lead one to expect 

 that a number of these character differences might be due to 

 more than one gene difference. We are seeing here, in the 

 segregation of unit phenotypic characters, the transfer of 

 blocks of genes, and it may be asked how these blocks come to 

 remain as units. 



When, on crossing two species, all degrees of intermediacy 

 are found in any character, we have a clear case of the breaking- 

 up of one of the gene blocks referred to. When, however, 

 the character acts as a unit, we do not know enough as yet to 

 affirm that only a single gene is necessarily involved. The 

 possibility of some unsuspected correlation mechanism cannot 

 altogether be dismissed. 



The recent emphasis on the idea of the multiple effects of 

 single genes also raises a difficulty. The result of postulating 

 multiple effects is to increase the number of genes which are 

 regarded as contributing to the phenotypic expression of any 

 one character. But, evidently, the more independent genes are 

 concerned in the expression of characters, the more difficult it is 

 to explain the independent segregation of characters as units. 

 At the present moment this point has scarcely more than 

 theoretical interest, but we shall have to return to it (p. 177) in 

 our consideration of the validity of a unit-character analysis of 

 living animals. 



