242 HEREDITY AND EUGENICS 



or improvement of methods. But the history of 

 civiKsation has consisted in just such advances, which 

 lead in each case to a corresponding increase in yield 

 for a given amount of labour. The law of diminishing 

 returns, however, limits this 3^ield, both in agriculture 

 and in industry. This leads Carr-Saunders to the 

 important conception of an optimum number, which 

 he states in the following words (p. 200): " Since the 

 laws in general are applicable to all, there will be, 

 taking into account on the one hand the known arts 

 of production, and on the other hand the habits and 

 so on of the people at any one time in an}' given area, 

 a certain density- of population which will be the most 

 desirable from the point of view of return per head of 

 population. There will, in fact, under any circum- 

 stances always be an optimum number." If the 

 population is either above or below that number, 

 the return per head will be less. For an}' country 

 there is, then, an optimum population corresponding 

 to its agricultural and climatic conditions and in- 

 dustrial development. Increase in skill brings an 

 increasingly dense population and a larger income 

 per head, and so long as skill increases the optimum 

 density of population will go on increasing. To 

 Malthus the problem was one of the relative rate of 

 increase of population and food production. With 

 the conception of an optimum, it is one of densit}' of 

 population and the productiveness of industry. 



An optimum number, if attained in a population, 

 implies the maintenance of a standard of living. This 

 number appears to be usually approached, both among 

 primitive tribes and in civilised countries. The 

 optimum will mean greater or less density of popula- 

 tion, according to the fertility of the country in the 

 broadest sense and the skill or degree of civihsation 

 of its people. In a poor country like Patagonia, 

 inhabited by a primitive tribe, the optimum con- 



