2 THE PRINCIPLES OF HEREDITY 



under the general name of biology. The work has been done 

 chiefly by zoologists and botanists, but, probably, heredity 

 concerns medical men more nearly than any other body of 

 scientific workers. Evidence is not wanting that the period 

 of professional neglect is drawing to a close, and that, ere long, 

 a systematic knowledge of the laws which control the 

 reproduction of living beings will be considered as necessary 

 a part of the equipment for the struggle against disease as 

 anatomy or physiology. 



3. The material for the study of heredity already, in the 

 possession of medical men is so valuable that there can be 

 little doubt that both medicine and heredity have suffered 

 from the failure to collect and apply it in a systematic way. 

 A knowledge of the natural history of the being to whom he 

 devotes his life is essential to the medical worker. No zoologist 

 or botanist can possibly have a knowledge of any animal or 

 plant so detailed and extensive as the educated physician 

 possesses of his own species. No experiments devised by 

 man can rival in scope and magnitude, in duration and 

 stringency, the many and vast experiments conducted by 

 nature through the medium of disease. An attempt will be 

 made in the present work to systematize and apply a portion 

 at least of this rich mass of material. No doubt, through 

 lack of knowledge or of judgment, the writer will frequently 

 do less than justice to his subject. His task is one of con- 

 siderable difficulty and magnitude. But his very failures 

 may stimulate future workers to tread more successfully the 

 path that he seeks to indicate. If he is able to preserve the 

 interest of his readers during the first few chapters it should 

 be possible to sustain it afterwards. There is more for medical 

 men in the study of heredity than is commonly supposed. 



4. At the present day educated men are agreed that all 

 forms of life are related to one another through an ancestry, 

 which, in the case of the more divergent forms, is very re- 

 mote ; and that all the higher and more complex forms have 

 been evolved from lower and more simple forms. 1 The only 

 question really in dispute among people acquainted with the 

 facts is the method by which evolution has proceeded. The 

 correct elucidation of that method involves the discovery of 

 the right doctrine of heredity. 



1 Evolution within limits has been tacitly admitted by nearly all 

 peoples and creeds. Thus it is almost universally believed that all 

 races of mankind have sprung from a common stock. Human races 

 are now very divergent, and each race is especially adapted to the 

 environment in which it has spent its past. 



