DARWINISM AND GENETICS 



which is by no means lethal in an absolute sense, but is fatal to 

 it merely as a potential embryo-sac, and vis-a-vis a particular 

 competitor. Selection, therefore, must favour the individual which 

 fits the conditions of its environment better at all stages of develop- 

 ment. 



Thus we can establish two important generalizations; first, that 

 heritable differences occur between different individuals of any 

 group; and secondly, that, as a consequence of these differences, the 

 individuals enjoy different, yet characteristic, chances of surviving 

 and leaving offspring under the conditions of incomplete survival, 

 arisuig from both internal failure and external competition, that 

 exist in nature. As Darwhi and Wallace pointed out, natural 

 selection will work to eHminate some variants and to perpetuate 

 others. The theory of evolution by natural selection relates the 

 whole of evolution to this process. As conditions change and as 

 new variants occur new types will arise, better fitted, or adapted, 

 than their predecessors to survival in the reigning environment. 

 By virtue of these new adaptations evolutionary change will 

 constantly be going on : evolution is the sum of adaptation. 



Darwinistn and Genetics 



Genetics has strengthened and amplified Darwin's theory in both 

 of its basic relations : to variation and to selection. Darwin was able 

 to show that hereditary differences occurred between individuals 

 on the scale required by his views, but he was never in possession 

 of a theory of heredity capable of relating them to the action of 

 natural selection. 



This lack of knowledge of the mechanism oi inlieritance placed 

 Darwin in a dilemma about variation, to the solution of which 

 the whole of his work on Animals and Plants under Domestication 

 was unsuccessfully devoted. He thought that inlieritance was 

 blending: that the contributions made by two parents to the 

 hereditary endowment of their individual offspring blended like 

 ink and water. Throughout posterity no separation was then 

 possible of the maternal and paternal elements. Thus variation was 

 always being lost, nay destroyed, to the extent of a half in every 

 generation of a randomly breeding population. Either evolution 



Elements oj Gaiclks ^73 ^ 



