HIGHWAY OF ANIMAL EVOLUTION 219 



tinctive local structures and conditions, necessarily go 

 distinctive local functions. 



Thus the features which distinguish this system 

 from all others are : ( i ) the multiplication of similar, 

 but not identical sets of organs in longitudinal and in 

 transverse series (metamerism) ; (2) The formation 

 of concentric strata and intervening spaces around a 

 central axis (germ layers and coelomic cavities) ; and 

 (3) the extension of the three great systems of convey- 

 ance throughout the body, parallel with the main axis of 

 growth, the alimentary in the middle, the nervous sys- 

 tem on one side, the vascular on the opposite one, with 

 various other organs arranged in symmetrical pairs on 

 the right and left. A more perfect system for differen- 

 tial functioning is inconceivable, for all agents, contents, 

 or commodities of each system are inevitably subject 

 to gradational treatment, selection, or distribution, 

 along each one of the infinite number of gradient lines 

 in the system, no two of which are alike. 



As growth proceeds, it is always possible to mod- 

 ify or improve cooperative action by local increase, or 

 diminution, at such points as the increasing exigencies 

 of supply and demand may call for. 



It should be noted that on the basic necessity of this 

 point for point correspondence in different individual 

 organisms having similar architectural plans, rests the 

 whole science of morphology and physiology, making 

 it possible for us to "identify" corresponding structures 

 and functions in different kinds of individuals, and at 

 different periods of their development. 



