THE HIGHEK ORGANISMS 111 



and vegetable kingdoms, but in neither do such plas- 

 modia show the least tendency to differentiation of cells 

 into tissues such as compose the more complex organisms. 



The size of the organism bears very little relation to its 

 complexity. It is true that size affords opportunity for 

 differentiation, but some of the undifferentiated plas- 

 modia are large enough to be measured in centimetres, 

 while some highly differentiated and exceedingly com- 

 plex organisms like rotifers are visible only through the 

 microscope. 



In endeavoring to ascend from the simple to the com- 



FIG. 36. Orbitolitea. Ideal representation of a disc of complex type. 



(Carpenter.) 



plex we are confronted by certain conditions the full 

 force of which cannot be felt until the chapter upon 

 Divergence has been perused. One of the most impor- 

 tant of these is the extinction of many of the intermedi- 

 ate forms, so that just when we seem to be comfortably 

 started upon a series of easy anatomical gradations, we 

 are obliged to skip a number of steps and endeavor to 

 fill them with imaginary forms. Another and even 

 greater difficulty is found in the fact that the process of 

 differentiation is not uniform, specialization in certain 



