INTRODUCTION 



The title of this short chapter is not "Genetical Theory of Evolution" 

 or "Genetics and Evolution" but "Genetic Theory and Evolution." 

 The title indicates that I do not intend to discuss the genetic basis of 

 evolution or evolution as such. In this book we are concerned only 

 w^ith the question whether the theory of genetics leads to ideas on the 

 theory of evolution, or demands certain basic assumptions in regard 

 to evolution. I have previously treated theoretical genetics under the 

 headings of (1) the nature of the genie material, and (2) the action 

 of the genie material. Thus the present problem is whether the views 

 on the nature of the genie material and its action, as developed here, 

 lead to definite conclusions regarding the theory of evolution. 



In order to see at which points theoretical genetics affects our 

 views on evolution, let us review briefly the elements involved, with- 

 out details. The classic theory of the gene from Mendel, De Vries, 

 Correns, Bateson, and Johannsen to Morgan gave a definite meaning 

 to what Darwin had called variation. Genie mutation became the 

 cause of hereditary variation; and genie recombination, in the widest 

 sense, the manner in which variation was effected. Selection became 

 the isolation of definite genie combinations. The continuous variation 

 with which Darwin worked (though his proofs for selection taken 

 from domestication were based largely upon discontinuous variation) 

 became a matter of phenotype, while genetic variation was dis- 

 continuous. Such a system makes it rather difficult to lead evolution 

 beyond the reshuffling of the mutated genes, that is, limited per- 

 mutation or variations on the same theme. The early Mendelians felt 

 this rather keenly and assumed a skeptical attitude toward evolution- 

 ary speculations, which found its extremest expression in Bateson's 

 Austrahan address ( 1914 ) with the embarrassing idea of evolution by 

 loss of inhibitors. 



At this time a genetic attack upon the problem had already been 

 attempted. Punnett (see 1915) and his student Fryer analyzed genet- 

 ically a case of major evolutionary adaptation, mimetic polymorphism, 

 which they found to be based upon Mendelian differences (as de 

 Meijere had done before). This led Punnett to ask how such differ- 

 ences would be distributed in interbreeding populations and how 

 selection could -"^rk on them. The mathematician Hardy worked out 

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