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THE AMERICAN NATURALIST [Vol. XLIX 



concentration of this substance in selachian blood — than in 

 similar solutions without the urea. 



V. General Considerations and Summary 

 The higher organisms have, therefore, developed a sys- 

 tem of regulation by means of which internal conditions 

 are kept relatively constant. This mechanism consists 

 essentially of a physical means of distribution of material 

 and heat — the circulatory organs and fluids — whose com- 

 position varies within narrow limits, a muscular, a glandu- 

 lar and a nervous mechanism for regulating the tempera- 

 ture, and a system of excretory organs for removing the 

 waste products from the circulating fluids. Both chemical 

 and nervous mechanisms of coordination are involved. 

 The variations in the composition of the circulating fluids 

 are such as will provide greater quantities of easily utiliz- 

 able material at a time when it is needed. The internal 

 secretions are important agents in maintaining the organ- 

 ism at a high pitch of efficiency through their influence 

 upon the neuro-muscular apparatus and the general me- 

 tabolism of all the tissues and organs. Eegardless of the 

 variations in external conditions, so long as these do not 

 transcend the limits within which life is possible, and 

 barring physical accidents or disease, the internal mech- 

 anisms keep it always fit, whether for work or rest, for 

 battle or for play. 



We have heard much about the survival of the fittest, 

 and about the role of the strong jaw and powerful teeth 

 and other physical characteristics in the struggle for 

 existence. The doctrine of evolution, so far as its morpho- 

 logical side is concerned, may be regarded as fairly well 

 founded. A little reflection, however, will show that the 

 morphological aspect is only one phase of the problem. 

 What profits it an animal to possess strong muscles and 

 sharp teeth unless these muscles shall be at all times ready 

 to contract quickly and surely? What if it become en- 

 gaged in combat with an adversary and its muscles be 



