6 INTEGRATION. 



however but parts of one body ; and in order that they may be 

 true members working harmoniously for the good of the whole, 

 and not isolated masses, each serving its own ends only, they need 

 to be bound together by coordinating bonds. Some means of 

 communication must necessarily exist between them. In the 

 mobile homogeneous body of the amoeba, no special means of com- 

 munication are required. Simple diffusion is sufficient to make 

 the material gained by one part common to the whole mass, and 

 the native protoplasm is physiologically continuous, so that an 

 explosion set up at any one point may be immediately propagated 

 throughout the whole irritable substance. In the higher animals, 

 the several tissues are separated by distances far too great for the 

 slow process of diffusion to serve as a sufficient means of commu- 

 nication, and their primary physiological continuity is broken by 

 their being imbedded in masses of formed material, the product of 

 the indifferent tissues, which being devoid of irritability, present 

 an effectual barrier to the propagation of molecular explosions. It 

 thus becomes necessary that in the increasing complexity of animal 

 forms, the process of differentiation should be accompanied by a 

 corresponding integration, that the isolated tissues should be made 

 a whole by bonds uniting them together. These bonds moreover 

 must be of two kinds. 



In the first place there must be a ready and rapid distribution 

 and interchange of material. The contractile tissues must be 

 abundantly supplied with material best adapted by previous elabo- 

 ration for direct assimilation, and the waste products arising from 

 their activity must be at once carried away to the metabolic or 

 excretory tissues. And so with all the other tissues. There must 

 be a free and speedy intercourse of material between each and all. 

 This is at once and most easily effected by the regular circulation 

 of a common fluid, the blood, into which all the elaborated food is 

 discharged, from which each tissue seeks what it needs, and to 

 which each returns that for which it has no longer any use. The 

 carrying on such a circulation of fluid, being in large measure a 

 mechanical matter, needs a machinery, and calls forth an expendi- 

 ture of energy. The machinery is supplied by a special con- 

 struction of the primary tissues, and the energy is arranged for 

 by the presence among these of contractile and irritable matter. 

 Thus to the fundamental tissues there is added, in the higher 

 animals, a vascular bond in the shape of a mechanism of 

 circulation. 



In the second place, no less important than the interchange of 

 material is the interchange of energy. In the amoeba the irritable 

 surface is physiologically continuous with the more internal proto- 

 plasm, while each and every part of the body has automatic 

 powers. In the higher animal, portions only of the skin remain 

 as eminently irritable or sensitive structures, while automatic 

 actions are chiefly confined to a central mass of irritable nervous 



