110 THE PRINCIPLES OF HEREDITY 



The surgeon sees mutilations every day, both those due to 

 accident and his own knife and those which occur in many 

 successive generations, such as pierced ears. He is constantly 

 employed in remedying great variations, progressive and 

 regressive, as tumours, cleft palates, and hernias. The phy- 

 sician notes the effects of diet and of various conditions of 

 life. He watches maladies which affect only part of the body, 

 as many skin complaints ; or which, from a focus, flood the 

 system with virulent toxins, as diphtheria and tetanus ; or of 

 which the microbes as well as the toxins invade every part of 

 the body, as syphilis. Some of these maladies are short and 

 sharp in action, as chicken-pox and influenza. In others the 

 period of suffering may be very prolonged, as tuberculosis 

 and leprosy. Many diseases, which have afflicted certain 

 races for hundreds, perhaps thousands of generations, have 

 only recently been introduced to other races. Biologists 

 have performed many experiments on animals and plants ; 

 but no experiments conducted by man can rival in duration, 

 magnitude, scope, and stringency these vast experiments 

 conducted by Nature. 



178. Speaking practically all this opulent field of labour 

 has been allowed to lie fallow. The problems of heredity 

 and evolution have not greatly interested the mass of medical 

 men, and disease lies outside the spheres of botanists and 

 zoologists. No doubt there has been much mention of 

 heredity by medical men, and much mention of disease by 

 naturalists ; but medical men have not as a rule bestowed 

 that close attention on heredity which they have devoted to 

 disease, and naturalists have certainly not applied to disease 

 those exact methods of research which they have applied to 

 other problems of heredity. The two great sections of bio- 

 logical workers, medical men and naturalists, have lived, as it 

 were, in separate compartments, and, like the adherents of 

 different religious systems, have abjured the works of one 

 another. Yet in this great field of labour might have been 

 found the answers to more than one important problem, the 

 means of settling more than one long-enduring controversy. If 

 acquired characters are transmissible, no matter how " faintly 

 and fitfully," the traits impressed on many successive genera- 

 tions by disease should become observable in the race. If ever 

 external influences affect the germ-plasm, the descendants of 

 those races which have been long affected by such diseases 

 as bathe the germ-cells with toxins or invade them with 

 micro-organisms should show the trace. If ever Natural 

 Selection has been at work we should see its processes de- 



