DARWINISM AND GENETICS 



diagrammatically in the relations of bacterium and bacteriophage 

 described by Delbriick and Luria, and is also very simply 

 demonstrated by the recent liistory of a wide range of crop 

 plants. 



A second class of changes leading to selective adjustment are 

 those in other genes of the same individual or race. Selection is a 

 means of fitting the genes to one another. If one gene in a population 

 is changed, all the others are exposed to new conditions of selection. 

 Thus, as we saw, a culture of "eyeless" Drosophila becomes modified 

 after a few generations. Polygenic modifications are selected 

 which prevent the eyeless gene expressing itself so drastically. 

 The environment for any one gene includes, therefore, the other 

 genes. 



In this light we can see that the genie balance we have been 

 discussing must always be the product of selection. Good balance 

 will be one which has been exposed to selection under the established 

 conditions and is consequently adapted to those conditions. Bad 

 balance will be one which is not adapted to those conditions. Thus, 

 while genetical principles enable us to see the answers to many 

 questions of evolution, it is equally true that Darwin's principle of 

 natural selection shows us how the genetical property of balance 

 comes about. In the same way it shows us both the meaning and 

 the origin of genetic systems. We have seen how the breeding system 

 is genetically adjusted, and indeed Darwin was himself aware that 

 the individuals of a species stood in a special adaptive relation to 

 one another in reproduction. He was the first to show that the 

 enforcement of inbreeding on plants which naturally outbreed 

 resulted in a decline of vigour and fertility; but he was baffled by 

 the general problem of these adaptations. He saw that the sex-ratio 

 of a species must be selectively adjusted, but was unable to see how 

 the selection could be achieved ; for he was unable to make out how 

 adjustment of the sex-ratio, or indeed of any other property of the 

 breeding system, could benefit the individual — which indeed it 

 does not. 



This is the problem which we must now consider in order to 

 see why some species outbreed and some inbreed; why there is 

 sometimes a change from one to the other, or even the adoption 

 of some more drastic device. We must compare these systems 



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