174 THE VARIATION OF ANIMALS IN NATURE 



(3) A good many of the specific characters observed in a 

 genus may occur in different combinations amongst the various 

 species. In so far as we are justified in assuming that similar 

 characters in different species can be rated as fundamentally 

 the same, we may use these permutations as evidence of 

 independent segregation. Lutz (1924) has described a case of 

 this sort in the S. American stingless bees (Melipona). Kinsey 

 (1930) presents convincing evidence that short-winged forms 

 of Cynips have been repeatedly produced from long-winged 

 species. 



(4) Crosses between distinct species may provide convincing 

 evidence as to the essential independence of characters. 

 Sometimes the hybrids show an extraordinary intermixture of 

 the characters of the two parents. Less commonly some of 

 the characters tend to remain together and segregate in blocks. 

 It is not actually necessary to assume that the correlation 

 between characters segregating in blocks is of a different nature 

 from that between characters segregating independently. 

 It might be suggested that disharmonies during cell-division in 

 the hybrids make normal segregation impossible. 



We have hitherto spoken of specific characters as units 

 without considering their relation to a genetic basis. This 

 relation is of importance when we try to define the meaning 

 of the term ' independent segregation.' An initial complica- 

 tion in the discussion arises from our ignorance as to whether 

 apparently similar phenotypic characters in different indi- 

 viduals or species really are the same. We know from genetical 

 researches that superficially similar mutants are not necessarily 

 due to a mutation at the same locus. In dealing with the 

 mutant forms of a single species the question can be always 

 answered by making the appropriate crosses. But in the mass 

 of species such crosses have not been or cannot be made. An 

 individual aberrant in one specific character is not usually 

 recognised to possess theoretical interest until death has made 

 experiment impossible. The direct identification of similar 

 specific characters in different species is usually impossible, 

 owing to refusal to cross and to the rareness of hybrids. The 

 point we wish to make here is that practically no analysis of 

 specific characters in terms of genes is available. Sturtevant 

 (192 1, p. 1 19 and foil.) has shown that some of the mutations in 



