372 HUMAN BIOLOGY 



Progress in illumination engineering has probably reduced 

 the injury to the clerk, the factory operative, the student 

 by more nearly approaching an optimum quality and 

 quantity of light upon the near object or throughout the 

 hail and shop, so that a tendency towards equahzation 

 of this factor as a selective city disadvantage is undoubtedly 

 occurring. 



INSECTS 



One further environmental factor not included under 

 the term social or human relations is that of insects which 

 serve as a means of transmitting disease. Cities are certainly 

 at present favored beyond their country neighbors in 

 relative freedom from the fly and mosquito. This is due to 

 the reduction in the number of horses in cities following the 

 advent of motor transportation, to the great pains taken to 

 prevent fly breeding in and about stables and garbage dumps, 

 and to the inevitable destruction of mosquito breeding places, 

 when low land is fifled and drained in the process of reclama- 

 tion for housing, parks and industry. 



The urban malaria death rate in 1920 in the United 

 States was 0.9 per 100,000 population and the rural was 5.9. 



The body louse and the rat flea are potentially greater 

 hazards in cities than in rural regions but both are so 

 readily controlled by cleanliness and suitable building 

 construction and maintenance for the exclusion of vermin 

 that we may properly ignore them as environmental factors, 

 at least in the United States. It is true, of course, that in 

 many rural counties of California and adjacent states 

 the distribution of the flea {X. cheopsis) by the ground squirrel 

 and possibly other rodents constitutes a rural danger, 

 and it is recalled that typhus fever is widely reported from 

 small town and rural regions of Georgia and Alabama, 

 pointing to some still undetermined insect conveyor of 

 disease which apparently operates as well in small as in 

 large communities. 



PERSONAL CONTACT 



Certainly with the communicable diseases, particularly 

 those transmitted by discharges through the nose and 



