HIGHWAY OF ANIMAL EVOLUTION 231 



rather in chemical and physical changes and in many 

 remote functional adaptations, the result of which is 

 obvious enough, but the contributory agencies exceed- 

 ingly difficult, or impossible, to locate, or analyze. 



Briefly, and speaking in very broad terms, one of 

 the first great improvements affecting the social life of 

 the cell was in limiting the free access of sea water to 

 the interior of the cell community, using in its stead 

 a very similar chemical fluid, as a vehicle for its prod- 

 ucts. 



At first this forerunner of true blood was driven 

 aimlessly to and fro through irregular body cavities by 

 spasmodic bodily contractions. After a long period 

 of evolution, this service was performed by a 

 rhythmically contractile tube, or primitive heart, which 

 drove the blood in definite circulatory movements 

 through specially constructed channels leading to and 

 from definite stations. 



A common system of internal transportation was 

 thereby established, which supplied each cell with 

 nourishment from a common store and removed its 

 waste materials. These obvious improvements in the 

 circulatory system, with many others, finally delivered 

 these primitive animals from the limitations of the sea 

 and made life in inland waters, on land and in the air, 

 a possibility, and ultimately a reality. 



During other periods of evolution, covering many 

 millions of years, countkss other improvements from 

 time to time were made, to meet the increasing de- 

 mands of the growing body. New kinds of blood cells, 

 chemical agents, and organized bodies of the most va- 

 ried and subtle character were contributed by differ- 

 ent bodily organs to the basic fluids of the blood, mak- 



