426 THE INSTABILITY OF THE HOMOGENEOUS. 



must expect extremely little differentiation of parts where 

 the differences of position are but little determined — which 

 is just what we find. This negative evidence is 



borne out by positive evidence. When we turn from these 

 proteiform specks of living jelly to organisms having an 

 unchanging distribution of substance, we find differences of 

 tissue corresponding to differences of relative position. In 

 all the higher Protozoa, as also in the Protopliyta, we meet 

 with a fundamental differentiation into cell-membrane and 

 cell-contents; answering to that fundamental contrast of 

 conditions implied by the terms outside and inside. On 



passing from what are roughly classed as unicellular organ- 

 isms, to the lowest of those which consist of aggregated cells, 

 we equally observe the connection between structural differ- 

 ences and differences of circumstance. Negatively, we see 

 that in the sponge, permeated throughout by currents of sea- 

 water, the indefiniteness of organization corresponds with 

 the absence of definite unlikeness of conditions : the periph- 

 eral and central portions are as little contrasted in structure 

 as in exposure to surrounding agencies. While positively, 

 we see that in a form like the ThalassicolZa, which, though 

 equally humble, maintains its outer and inner parts in per- 

 manently unlike circumstances, there is displayed a rude 

 structure obviously subordinated to the primary relations of 

 centre and surface: in all its many and important varieties, 

 the parts exhibit a more or less concentric arrangement. 



After this primary modification, by which the outer tis- 

 sues are differentiated from the inner, the next in order of 

 constancy and importance is that by which some part of the 

 outer tissues is differentiated from the rest; and this corre- 

 sponds with the almost universal fact that some part of the 

 outer tissues is more exposed to certain environing influences 

 than the rest. Here, as before, the apparent exceptions are 

 extremely significant. Some of the lowest vegetal organ- 

 isms, as the Hematococci and Protococci, evenly imbedded 

 in a mass of mucus, or dispersed through the Arctic snow, 



