DIFFERENTIATION OF STRUCTURE 29 



are controlled by muscles. This leads naturally to the 

 appendages of the vertebrates, which appear in a variety 

 of forms wings, legs, arms, and flappers. 



Food must be distributed to all parts of the organism. 

 This is done in the uni-cellular forms by the food enter- 

 ing the cell and being digested there, but as soon as a 

 body is formed this is impossible. Among the lower 

 forms of plants and animals this is achieved by a passing 

 of food from cell to cell, but this is not sufficient for the 

 more complex forms. Plants provide themselves with 

 vascular bundles by means of which there is a con- 

 tinuous circulation of food materials. The next step 

 among animals is the formation of canal-like spaces in 

 the body, usually merely outgrowths of the digestive 

 cavity, which distribute the digestive food materials. 

 Next we come to the formation of a series of vessels 

 which are at first closely associated with the alimentary 

 canal, and in these vessels there is a constant circula- 

 tion, at first as it were undecided as to course, but 

 ultimately taking up a definite direction of flow. The 

 next step is the acquisition by some portion of these 

 vessels of a power of contraction and expansion, and 

 this portion takes upon itself the control of the pro- 

 pulsion which was at first caused by the muscular 

 contractions of the body. Then comes the development 

 of minute branches of these vessels, the capillaries. 



As the complexity increases there is a demand made 

 on the organism not only for a greater supply of this 

 material, but also for a more efficient type of material. 

 This means that the fluid, which in the lower forms 

 differs only from the ordinary fluid in which the animal 

 lives in containing more food material, must alter, which 



