^20 THE ORIGIN OF SPECIES 



Neither nineteenth-century biologists, nor those who opposed 

 the theory of evolution on rehgious grounds, saw the real weakness 

 of the Darwinian theory, which is the assumption that with 

 selection would go a progressive change in the limits of variation. 

 No evidence for this was given, and even if it had been, a complete 

 theory would still have needed to find some explanation of how 

 it was brought about ; we shall return to this last point later. 

 In fact we now know that in the form in which Darwin made it, 

 the assumption is untrue. Much ' continuous variation ', on 

 which alone he based his theory, we now know to be not inherited 

 at all ; it is phenotypic only, and caused by environmental 

 influences in each generation. That part of continuous variation 

 which is genotypic (and largely caused by numerous genes 

 of small effect, or polygenes) could only be selected until the 

 genes were all homozygous ; beyond that point selection could 

 produce no further change. 



In spite of these difficulties, the modern theory of evolution 

 is based on natural selection, but it also takes account, as Darwin 

 was unable to do, of Mendehan and post-MendeHan genetics. 

 Selection within a species is assumed to take the existing geno- 

 types as far as they can go, not only in favouring individuals 

 which are homozygous for valuable characters, but in fitting 

 particular genes into a set of other genes in the presence of which 

 they can exert their maximum effect ; in selecting, that is, a 

 favourable genotype On the whole, success in this direction is 

 likely to have been already achieved for any given species in 

 its natural environment, and the further role of selection is 

 then largely conservative ; it maintains the type, and only when 

 the environment changes, as with the melanic moths in industrial 

 England described above (p. 710), will it work instead so as to 

 change the type. 



MUTATION 



The action of selection would be different if new genotypes arose, 

 for these would be in competition with the old, and selection would 

 determine which survived or, if both survived, in what proportion. 

 In all the species which have been intensively studied, there is 

 evidence that new genotypes, or mutants as they are called, do 

 occasionally arise. The mutations, or nuclear changes which pro- 

 duce the mutants, are of three main types. Numerical mutations 

 are changes in the numbers of chromosomes, and structural muta- 



