THE TRACES OF ANCESTRY 



such as inversions, interchanges, and fragmentations, float in the 

 species until they happen to combine favourably with the gene 

 discontinuities. Two consequences follow from this accidental 

 character in the relationship. In the first place changes may take 

 place in chromosome structure or number, or in gene content, 

 without any obvious change in the phenotype. Two complementary 

 instances are offered by Drosophila and the bug Thyanta. D. simulans 

 and melanogaster are externally almost indistinguishable except to 

 one another. Genetically they differ by an inversion. In a species 

 of Thyanta, Wilson in 191 1 found two races, one with 7 and the 

 other with 12 pairs of chromosomes. Obviously these two races, 

 not previously distinguished, would be intersterile and they were 

 indeed described (after the event) as distinct species. 



Thus great structural and polygenic changes can go on under 

 the surface, as cryptic variation, without any differentiation of 

 external form. It follows that similarity of chromosomes is some- 

 times a worse guide to similarity of descent than is external form. 

 It also follows that it is a matter of pure chance whether the newer 

 type of gene effect is combined with the new kind of chromosome 

 character or not. It may as well be that the primitive morphology 

 is combined with the new chromosome number as the reverse. 

 In the second place a particular structural difference that we observe 

 now as distinguishing two groups may, as we saw in Drosophila, 

 not be the one which actually helped to separate them. It may 

 stiU have been present in both at the time they were separated by 

 some other agency of which we can no longer uncover the traces. 



Let us not, however, suppose that the systematist may safely 

 proceed unguided by breeding experiment or chromosome analysis. 

 Too often he feels compelled on the one hand to regard as a hybrid 

 an individual or a type which is intermediate between two other 

 arbitrary types from a heterogeneous mating continuum. And, on 

 the other hand, he feels compelled to take as a variety an individual 

 or a type which represents merely a marker gene, whether freely 

 floating in such a continuum, or attached to a super-gene or a 

 chromosome variation or an ecological situation. These errors he 

 can avoid by combining the rules to be derived from external form 

 and geographical distribution with those to be derived from breeding 

 behaviour and chromosome variation. This combination has yet to 



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