CHAPTER IY. 



THE INFLUENCE OF TEMPERATURE. 



THE sun, the source of light, supplies other powerful stimuli 

 to organic life on our globe. All the heat which influences 

 the development and continuity of life either is now, or formeily 

 was, derived from the sun, in whose rays light- and heat-rays 

 exist in combination. The influence of the heat-rays only, on 

 animal life and on its distribution on the globe, will form the 

 subject of the present chapter. 



It must be almost superfluous to bring forward any special 

 facts to prove that heat, or the degree of temperature at a given 

 time, has a marked influence on the life of animals and on their 

 vital functions. Everyone knows that perspiration, i.e. the 

 action of the sweat-glands in the skin, increases as the tempera- 

 ture rises ; and that a considerable heat must be kept up if a hen's 

 egg is to be perfectly developed or hatched. The heat which 

 reigns during summer in the Eastern States of America, in 

 Madrid, Naples, and other places, is often intimately connected 

 with fatal epidemics, nay, sometimes is productive of them. Most 

 Europeans become indolent and slothful if they are forced to 

 pass the hot season between the tropics or even in Naples or 

 Madrid. The approach of winter, on the other hand, is equally 

 perceptible, and it may be confidently asserted that many millions 

 of htirnan beings make their livings and support themselves 

 solely by the indirect results of this transition from summer 

 warmth to winter cold, otherwise frequently so injurious. If, for 

 instance, we suppose that the thirty or forty millions of human 

 beings who pass through the severe cold of an American winter 

 were by some means relieved of the necessity of buying, say every 



