Fitness in the Living World 37 



tenths of a degree. Anyone who has ever tried to maintain 

 an incubator at constant temperature, and especially under 

 greaJ external changes, will be in a position to appreciate 

 the remarkable efficiency of this living thermostat. In hiber- 

 nating animals the body temperature falls during the winter 

 sleep, and in severe weather it may continue to fall until it 

 approaches the freezing point. The animal then wakes up, 

 as if an automatic alarm had been set to rouse it when the 

 danger point had been reached, and by means of muscular 

 movements, increased respiration and oxidation, and some- 

 times by feeding, the temperature is raised. This safety 

 device is found not merely in the highest warm-blooded 

 animals, such as hibernating bears, but also among some 

 insects, such as bees. When the temperature in the winter 

 cluster of bees goes below 57 F. the bees become active, 

 eat honey, oxidation increases and the temperature of the 

 cluster rises. 



The respiratory mechanisms by which oxygen is brought 

 into contact with every particle of living substance and the 

 exhaust gas is eliminated are far more perfect than is to be 

 found in any other engine. In the case of insects, minute air 

 tubes or tracheae run to every part of the body, whereas in 

 many other animals in which respiratory organs (gills or 

 lungs) are limited to certain regions, the blood contains a 

 substance, haemoglobin, which serves as a wonderfully effi- 

 cient oxygen carrier. 



The mechanisms of living things for obtaining, preparing, 

 absorbing, and utilizing substances as fuel are incomparably 

 more complex and efficient than in any engine of human 

 devising, and the utilization of foods for growth and repair 

 is wholly unparalleled in any other mechanism. 



The uniqueness of the living machine is nowhere more 

 evident than in its capacity for reproduction. Imagine any 



