2 HOMOIOTHERMISM 



the other hand there are in parts of the tropics and in 

 warm springs certain animals which inhabit an environ- 

 ment that never falls below about 27 deg. C. Lusk (1917) 

 suggests that homoiotherms probably had their origin 

 along the shores of tropical oceans, where they lived in 

 an environment which continually had a rather high 

 temperature to which they gradually became adjusted. 



Metabolism requires a continual supply of energy and 

 under usual conditions homoiotherms need more energy 

 from day to day than poikilo therms. However, the change 

 from the poikilothermic to the homoiothermic condition 

 has not been related directly to a higher energy require- 

 ment, but to the use of new mechanisms for tempera- 

 ture regulation and better insulation. The protoplasm 

 of the two types of animals is similar in its energy re- 

 quirements, but poildiotherms are able to live at slower 

 rates at lower temperatures, and homoiotherms, with the 

 exception of certain hibernators which become quite in- 

 active, are not. The homoiotherms have gained a great 

 advantage in that they are able to live to practically their 

 full capacity at all times, but they have also been obliged 

 to assume certain limitations in order to attain this end. 

 Their method of living requires the continual expenditure 

 of a considerable amount of energy and the control of 

 their metabolic processes has to some degree passed 

 under the domination of various controlling mechanisms. 

 If there is lack of correlation between these controlling 

 mechanisms, the result may be death. The mechanisms 

 for heat production must operate under rather accurate 

 control and must be correlated with mechanisms for heat 

 dissipation. If the former run ahead of or behind the 

 latter, fever or subnormal temperatures result. 



Homoiothermic animals have become adjusted more 

 and more to uniformity in the conditions for metabolism. 



