THE IMPORT OF PROTOPLASM. 427 



the indiflerent tissues, which, being devoid of irritability, present an 

 eliectual barrier to the propagation of molecuhir explosions. It thus 

 becomes necessary that, in the increasing complexity of animal forms, 

 the process of differentiation should be accompanied by a correspond- 

 ing iutegration, 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 elaboration 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 inter- 

 course of material between eacli 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. 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 construction 

 of the primary tissues, and the energy is arranged for by the presence' 

 among these of contractile and irritable matter. Thus, to the funda- 

 mental 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 animals portions only of the skin remain as eminently 

 irritable or sensitive structures, while automatic actions are chiefly 

 confined to a central mass of irritable or nervous matter. Both forms 

 of irritable matter are separated by long tracts of indifferent mate- 

 rial from those contractile tissues through which they chiefly manifest 

 the changes going on in themselves. Hence the necessity for long 

 strands of eminently irritable tissue to connect the skin and contrac- 

 tile tissues, as well with each other as with the automatic centres. 

 Similar strands are also needed, though perhaps less urgently, to con- 

 nect the other tissues with these and with each other. To tlie vascu- 

 lar bond there must be added an irritable bond, along the strands of 

 which impulses, set up by changes in one or another part, may travel 

 in determinate courses for the regulation of the energy of distant 

 spots. In other words, part of the irritable tissues must be specially 

 arranged to form a coordinating nervous system. 



Still further complications have yet to be considered. In the life 

 of a minute homogeneous amoeba, possessing no special form or struct- 

 ure, there is little scope for purely mechanical operations. As, how- 



