CHAPTER XI 

 HUMAN DISEASES 



Neglect of the human species by modern students of heredity The meaning 

 of racial change Progression and retrogression The present evolution of man 

 Microbic disease Air, water, earth, and insect-borne diseases Toxins Species 

 that produce abundant extra-cellular toxins, and species that produce little or 

 none Phagocytes The duration of disease and its cause Acquired immunity. 



F 



357. "| ^ORMERLY man, considered as a whole, as a bundle 

 of useful and related characters, occupied a good deal 

 of scientific attention. Darwin traced his descent. 

 Huxley sought to fix his place in nature. Spencer, Romanes, and 

 Lewes dealt with his mental evolution mainly from the stand- 

 point of Lamarckism. But of late years the turn given by fashion 

 to biological work has been unfavourable to an adequate study of 

 him. He cannot be dealt with experimentally, at any rate not by 

 inter-varietal crossings arranged by the worker. The results of 

 crossings arranged by nature have not to my knowledge attracted 

 much attention except in so far as is indicated by the surprising 

 statement that human skin-colour is the only feature which crossed 

 varieties blend perfectly. The alternative inheritance of various 

 human abnormalities has been noted. Biometricians have devoted 

 rather more attention to man than Mendelians and mutationists. 

 They have endeavoured, on the one hand, to ascertain the degree 

 of correlation between certain of his parts, and on the other to 

 discover the extent to which some of his variations tend to be 

 inherited. Only a very small portion of the total field has been 

 covered, however, and some biometric work is vitiated by a 

 failure to distinguish adequately between ' innate ' and ' acquired ' 

 characters. 1 The importance of drawing this distinction will 

 become manifest as we proceed. 



358. While studying heredity in connection with human beings 

 we shall have to depend largely on patent facts quite indisput- 

 able facts, however collected by simple observation, and largely 

 also on the special method of comparing races. The data left 



^See 706 et seq. 



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