4 THE CAVENDISH LECTURE 



both those attempts to improve the racial qualities of future 

 generations. No, I am thinking of something far wider and 

 broader tlian this ! The public health service brings a man right 

 up against the fundamental biological problems of society. He 

 is forced to consider them ; he cannot progress until he has 

 determined whether nature or nurture contributes more to the 

 evils he sees around him. The physician in general practice has 

 one main object —the welfare of the individual human being who 

 chances to be his patient ; the relief of individual suffering and 

 the prolongation of lives which possibly have little or no social 

 value are the primary duties before him. But in the public 

 health service the medical man must see individuals in the mass. 

 P'or example, within his district he finds a certain intensity of 

 infant mortality. Is it due to employment of mothers, to dearth of 

 breast-feeding, to unhealthiness of parents, to alcohol, to syphilis, to 

 overcrowding ? Or again in what proportions is it due to one or 

 more of these causes ? He turns to thousands of school-children, 

 and at once recognises among them factors which render them 

 racially inefficient — congenital cataract, defective teeth, epilepsy, 

 feeble-mindedness, evidences of active or healed tuberculous 

 lesions; he is at once impelled to inquire into the causes of these 

 things, and from that moment he is a eugenist in the true sense 

 of the term. In the words of Sir Francis Galton, he begins to 

 study the agencies under social control which may improve or 

 impair the racial qualities of future generations, either physically 

 or mentally. The whole range of eugenic problems is opened up 

 for him at once. He has an entire city, a county at his disposal, 

 and data in quantity beyond the dreams of the most avaricious 

 statistician, but what can he do with it ? What is it possible for 

 him to make of it ? 



He is given a laboratory on a scale unparalleled in pure 

 science ; he has before him a population being born, breeding, and 

 dying, which the most keen experimental disciple of genetics must 

 envy him. He has his thousands, and he is able to record fine 

 differences in parentage and environment which must escape those 

 of us who can only breed our tens of dogs or hundreds of mice or 

 guinea-pigs; we can only discriminate crude differences, such 

 as colour of coat, or length of hair, or shape of nose ; or environ- 

 ments which are measured in rough differences of temperature, of 

 light, or of bulk of food. What differentia are there that we can 

 observe in the intelligence of mice ? Even with the dog relative 



