3 io THE VARIATION OF ANIMALS IN NATURE 



of its operation that attention has been concentrated. As a 

 result, when opinions differ, as they often do on this topic, 

 there is no body of crucial evidence to which we can appeal. 



Though we are primarily interested in establishing whether 

 or not a selective process actually occurs in nature, we are 

 also concerned in the secondary question, whether Natural 

 Selection, if operative at all, has played the main part in the 

 evolution of the lower taxonomic categories. We have treated 

 under four headings the data which enable us to form some 

 opinion as to the answers to these questions. 



We first deal with selection under artificial conditions. 

 The discovery of the pure line is one of the major contributions 

 of the geneticist to evolutionary theory and has revolutionised 

 our ideas as to the significance of the superficially bewildering 

 array of phenotypes. As a general rule, selection in any one 

 direction appears soon to reach a definite limit beyond which 

 progress depends on the occurrence of further mutations. 

 It is not possible to define how circumscribed these limits are, 

 but we no longer feel able to assume the existence of the uni- 

 versal storehouse of variation on which Darwin thought he 

 was at liberty to draw. The evolution of domestic animals, 

 during which the original types have undergone great modi- 

 fication, appears to have little in common with the normal 

 course of evolution. The stock of variants has probably been 

 greatly increased by the crossing of more than one wild species, 

 while the strict isolation of different forms from one another 

 and the selection for pedigree rather than for phenotypic 

 quality have little counterpart in nature. 



Secondly, we have considered the direct evidence for a 

 selective process in nature. We have shown that no demon- 

 stration of large, apparently random, mortality can reveal 

 whether selection is operative or not. It is the small percentage 

 of selective deaths which is significant, not the random death- 

 rate, even if this is extremely high. If the death-rate is largely 

 random, this may slow down the spread of rare, beneficial 

 mutants, but it cannot permanently inhibit it, provided they 

 really have a greater chance of survival and reproduction. 



The direct evidence for the occurrence of Natural Selection 

 is very meagre and carries little conviction. In a few instances 

 there is some evidence for a selective process which in some 

 cases tends to promote the survival of the mean of the stock. 



