232 GRAND STRATEGY OF EVOLUTION 



ing the blood itself a veritable physical and chemical 

 laboratory. 



Finally the common blood stream was divided qual- 

 itatively into venous, lymphatic, and other special car- 

 riers; old stations of give and take were cut off and 

 others provided with increased facilities. And with 

 all these betterments in service, the pressure, the local 

 rate of flow, the nutritive contents, and the tempera- 

 ture of the blood were at last so artfully regulated as 

 to provide for each one of the countless millions of 

 bodily cells, according to their individual necessities, 

 the optimum conditions, apparently, for the perform- 

 ance of their functions. 



Many of these improvements were purely mechan- 

 ical devices, gross architectural readjustments of purely 

 material things, yet they are no more, or less, signifi- 

 cant in their constructive Tightness, than the less tan- 

 gible nervous and biochemical devices, or the altogether 

 intangible "vital" ones. 



It was through the agency of all these achievements 

 in internal conveyance, and many other contributory 

 services of a similar character, that the higher phases 

 of life, as we see it in birds and mammals, became a 

 possibility and then a reality; a life capable of the most 

 sustained, strenuous, and varied activities ; one that had 

 freed itself from the dominion of the sea and shore, 

 and was now freed from the dominion of the tropics, 

 and of the changing seasons; for at last life had cre- 

 ated within itself an approximately right environment 

 for its constituents, and the freedom of the earth and 

 the air was opened to them. 



4. The Nervous System. In the long history of 

 the arthropod-vertebrate stock, there has been a per- 



