No. 582] SIGNIFICANCE OF INTERNAL CONDITIONS 359 



sluggish from cold? Or, supposing the temperature to 

 be favorable, it be not able to control those muscles accu- 

 rately and sink its teeth into the vital spot of the enemy? 

 The answer is simple; another skeleton will soon lie 

 bleaching. Somewhere or other evolution must have been 

 concerned with the functional side. One protective mech- 

 anism has been suggested by the slow action of muscles 

 in the cold and their more rapid action at higher tempera- 

 ture. The combat between a dog and a snake may be a 

 fairly even one when the weather is warm, and very 

 much in favor of the dog when the weather is cold. There 

 is a strong presumption that the elaborate and compli- 

 cated nervous vascular and glandular mechanisms, some 

 or all of which are developed in birds and mammals, have 

 some bearing on the general problem of evolution. It has 

 rendered them far more independent of the environment 

 than poikilothermal animals are. There is not so much 

 necessity of hibernation during the winter, and a frosty 

 morning is as good as any for hunting. 



And if we consider that the changes of energy and 

 material underlie all the other changes in the organism, 

 regardless of the source from which they arise, it will be 

 apparent that at least one part of the final discussion of 

 evolution will be in terms of the changes of matter and 

 energy within the organism. 



The problems of the general processes of evolution — 

 the adjustment of the animal to its environment or re- 

 sponses to changes in it, variation, adaptation, heredity 

 and geographical distribution, and even the biochronic 

 equation (De Vries) may all be approached from the point 

 of view of the experimental physiologist. The considera- 

 tion of these subjects will be taken up in subsequent 

 papers. 



