PART THREE 



Genetics and Evolution 



That biological organisms have evolved has long ceased to be a theory, 

 and has become a generally accepted fact, which itself requires an 

 explanation. In the formation and testing of an explanatory hypothesis, 

 genetics must collaborate with systematics and ecology. There is no 

 space here to summarize the relevant data from the two latter sciences, 

 but in the first chapter a short account is given of the most definite 

 evidence concerning the nature of evolutionary change. The next 

 chapter deals with the analysis, in genetical terms, of taxonomic dif- 

 ferences; this is perhaps the most valuable contribution which genetics 

 has yet made to the study of evolution. These two chapters pose the 

 problem which a theory of evolution has to solve; the attempts to solve 

 it, mainly by means of Darwin's theory of natural selection translated 

 into modem terms, are discussed in the last chapter. 



