LECTURES IN BIOLOGICAL SCIENCES 



understand and control his own nature, that is to say, the sys- 

 tems of religion, philosophy, and politics. These are fields of 

 human endeavour in which emotion is very powerfully at work. 

 In tracing the history of Darwin's scientific theories from his 

 day to our own, it is not surprising that we generally find that 

 a worker who may make a valuable contribution in a certain 

 area at the same time commits himself to statements which are 

 less satisfactory in other parts of the enormously complex and 

 emotionally charged field to which evolution is relevant. There 

 are very few, if any, authors whose works we can accept in their 

 entirety. Darwin himself escapes much criticism on this score, 

 because he confined himself to establishing the fundamental 

 main lines of his argument. He did not attempt— and indeed, 

 owing to the backward state of the understanding of hereditary 

 phenomena in his day, was not in a position— to discuss the pre- 

 cise way in which it affects human political and religious thought. 

 The century since the publication of the Origin of Species has 

 seen the rise and development of the science of genetics, and it 

 has been only gradually, through the endeavours of many au- 

 thors, each contributing something of value, but also something 

 which should be rejected, that we have been working towards 

 a true appreciation of how the theory of evolution should be 

 formulated in terms of our new understanding of biological in- 

 heritance. 



As is well known, the foundations of our understanding of 

 heredity were laid when Mendel demonstrated that the charac- 

 ters of an organism are developed on the basis of certain hered- 

 itary potentialities which it receives from its parents, and further 

 that these potentialities are carried in separable, discrete, hered- 

 itary factors— nowadays known as genes. It was not till about 

 1900 that this theory became widely known in the scientific 

 world. Immediately many sets of experimental data were ob- 

 tained which could be interpreted according to the Mendelian 

 rules. There was for some time, however, a controversy as to 

 whether Mendelian heredity is the most general or even the 

 exclusive mode of biological inheritance, or whether it is a 

 more special type of less importance. The older and alterna- 

 tive theory is that of blending inheritance. It supposes that the 

 hereditary contributions from the two parents come together 



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