AND ITS INHABITANTS 157 



merable indirect effects through inheritance and material re- 

 sources. In this he appears to be like all other organisms. 

 For every species of living being there is apparently a certain 

 optimum or most favorable condition. As the temperature, 

 humidity, or other climatic elements depart more and more 

 widely from the optimum, the animal's reproductive rate 

 diminishes, its general strength declines, it becomes more 

 susceptible to disease, and its life is shortened. Between the 

 optimum conditions and those which cause death there may be 

 a wide range in which life and even health are possible, but 

 in which the organism is not at its best. 



One of the great lessons of biology is that man's physio- 

 logical nature is essentially the same as that of the lower 

 animals. A law which applies universally to them applies also 

 to him. This is eminently true in respect to climate. By 

 measuring the rate of reproduction of infusoria, the rate of 

 growth of plants, the amount of oxygen consumed by crayfish, 

 or the length of life of the boll weevil we can arrive at an exact 

 estimate of the effect of climate upon these various organisms. 

 In the same way we can measure man's response to climate and 

 find out just what conditions are the best and how much harm 

 is done by departures from the ideal. Thus far man's physical 

 relation to climate has been tested chiefly in the following 

 ways : ( i ) by frequent measurements of the weight of healthy 

 persons or of those suffering from tubercular or other diseases, 

 (2) by daily or weekly tests of people's strength, (3) by 

 examination of the amount of work done by specially chosen 

 subjects or by piece workers in factories day after day 

 throughout long periods, (4) by measuring the amount of 

 carbon dioxide given off in the breath of people who are at 

 rest under various climatic conditions, (5) by tabulating 

 people's estimates of their own feelings of comfort or dis- 

 comfort at different temperatures and humidities, and (6) by 

 ascertaining the number of deaths at different seasons of the 



