CHAPTER X 



Experimental biology continued. Mendelism and the mul- 

 tiple factor hypothesis. Human inheritance and eugenics. 



We have considered in a previous chapter the physical basis 

 of inheritance and have seen that the manifold characters 

 of organisms are probably determined by certain entities in 

 the cell, which are shuffled about at the time of maturation 

 like the cards in a pack or dice in a box, and recombined in 

 fertilization so as to produce entirely new combinations of 

 characters in the offspring 1 . In the present chapter we shall 

 consider the applications of these facts to inheritance in or- 

 ganisms in general and man in particular, with especial ref- 

 erence to eugenics or the. improvement of the human race. 



"While variation is the foundation of evolution, inheritance 

 may be likened to the keystone of its arch, without which* 

 permanence would be impossible. The truth of inheritance 

 has been recognized throughout the ages. Aristotle and other 

 early writers discuss it. "Plutarch mentions a Greek woman 

 who gave birth to a negro child, and was brought to trial for 

 adultery, but it transpired that she was descended in the 

 fourth degree from an Ethiopian. ' ' 1 



Many types of inheritance have been recognized in the past 

 (reversion, atavism, telegony, particulate, blending, etc.), but 

 modern research points strongly to a single method, modified 

 indeed by many factors, but nevertheless uniform in its un- 

 derlying principle. So far at least as higher types of life 

 are concerned. When we come to the beginnings of life, to 

 the unicellular forms and the simpler metaphytes and 

 Metazoa, our knowledge is too limited to admit of anything 

 approaching generalization. In the unicellular forms the 

 mechanism of inheritance itself is evolving, and the type of 

 inheritance must therefore of necessity be indeterminate. 



The essential feature of Mendelian inheritance is not the 

 dominance of one trait over another, but rather the persist- 

 ence of identical traits from generation to generation (un- 



1 Ribot, "Heredity," p. 167. D. Appleton and Company. While 

 Plutarch's information was probably faulty, viewed in the light of 

 modern research, his statement ne\ 7 ertheless shows the belief of his day 

 in the controlling influence of heredity in human life. 



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