EVOLUTIONARY MECHANISMS 299 



on the argument that selection for a. character, such as is practised by 

 man in building up domestic varieties, should act in exactly the opposite 

 way and should favour modifiers tending to increase the expression of 

 the gene in heterozygotes. In a domestic animal like the fowl, one 

 therefore finds that genes from domestic races tend to be dominant 

 over the wild allelomorphs. By continued crossing, however, it should 

 be possible to free the genes from their domestic genotypic milieu and 

 get them into an otherwise wild-type background, when they should 

 show the usual recessiveness. A long series of crosses was made, and 

 the prediction confirmed. This proves that selection of the milieu is 

 possible, and that it occurs in artificial selection; it is not a complete 

 demonstration that it also happens in selection in Nature. 



Modification of the genetic background, if it is considered a possi- 

 bility, could be invoked to explain several difficulties in evolutionary 

 theory. For instance, we find cases of mimicry in which the model and 

 mimic species resemble one another in many details, and yet differ 

 only by a single gene. It is very difficult to beUeve that a random gene 

 mutation has produced such a compHcated resemblance. But we can 

 suggest that the mutation at first produced only a crude likeness which 

 has been refined and perfected by the gradual modification of the 

 milieu. Similarly the insensible gradations found in trend-evolution 

 may have been produced not by selecting a long series of slightly 

 different allelomorphs, but by a gradual modification of the background 

 against which only a few major genes were working. 



8. Evidence for the Occurrence of Natural Selection 



Natural selection is a necessary consequence of hereditary variation 

 in fitness; the inevitability of the process is so obvious that Darwin was 

 able to use it to convince the world not only of the mechanism but, 

 more important, of the fact of evolution. Competition and natural 

 selection between species is also a common fact of observation, although 

 rather Httle exact quantitative work has been done on it. Gause^ is 

 studying the matter in the simple case of competition between two 

 protozoon species in culture, and TimofeefF-Ressovsky^ has described 

 the competition between D. melanogaster and D.funebris. 



Natural selection between varieties of a species,^ which is the kind 

 which is important for evolution, has been even less studied, either in 

 Nature or in the laboratory. Harrison^ has described an interesting case. 



^ Gause 1934. ^ Timofeeff'-Ressovsky 1933. 



^ Rev. Robson and Richards 1936. * Harrison 1920. 



