92 WORLD-POWER AND EVOLUTION 



produce the most marked effect in summer, a very strong but less 

 marked effect in the spring, somewhat less in autumn, and least 

 in winter. As yet we cannot say positively what this means. 

 Two possibilities suggest themselves. One is that people have 

 less power of resistance in summer, just as we have found to be 

 the case in mild climates when these are compared with those where 

 changes are abundant. It is a well-known fact that the varia- 

 bility of the temperature from day to day is much less in summer 

 than in winter. The other possibility is that change of tempera- 

 ture has less effect in winter than in summer because in cold 

 weather we protect ourselves from the outside air by means of 

 our houses. The fact that we are dealing with sick people and 

 not with those in good health makes our results all the more 

 remarkable. Remember that sick people do not go out of doors, 

 and that the deaths which we are studying occurred before the 

 value of fresh air had begun to be realized as it is today. Never- 

 theless these sick people felt at once the effect of a change of even 

 a single degree in the mean temperature. They felt it in winter as 

 well as in summer. Perhaps this means that a change in tempera- 

 ture is accompanied by some other change, perhaps electrical, 

 and that the other change is the important factor. However 

 that may be, there seems no escape from the conclusion that the 

 slightest change of temperature finds its immediate reflection in 

 man's health. 



Many of us have wondered why we have "spring fever" when 

 the first warm days arrive. We know that at such times the 

 temperature is much more favorable than formerly. Why, then, 

 should we feel a sense of lassitude? The answer is that the effect 

 of a constant temperature is very different from the effect of a 

 change to that temperature. An average of 64° for day and 

 night together is the most favorable, but a drop from 75* to 64" 

 is highly stimulating and causes a decline of perhaps 10 per cent in 

 the deathrate. A similar rise, on the other hand, from 53 to 64 , 

 causes a corresponding increase in the deathrate. 



