ELEMENTARY ORGANISMS : DIFFERENTIATION OF TISSUES. [) 



> 



ganism witli variously formed parts is the result. Each 

 part is originally a completely independent organism of 

 equal value with all the others, and each has, therefore, 

 been very aptly called an elementary organism. But 

 together with the modification in the form, a change 

 usually takes place in the qualities. Of the various 

 qualities possessed by the protoplasm in its original 

 form, some are lost, others are especially developed. 

 A colony of uniform elementary organisms may be 

 likened to a society in the lowest stage of civilisation, 

 in which each member still personally performs all the 

 tasks necessary to life ; but a composite organism, with 

 variously developed and modified elementary organisms, 

 may be likened to a modern state of which the various 

 members perform very different tasks. The more highly 

 developed plants and animals are of this sort. They 

 originate from a number of elementary organisms — or 

 cells, as they are also called — originally uniform; but 

 these develop in very different ways — differentiate, as 

 is technically said, and then acquire very different ap- 

 pearance and purpose. In some the power of causing 

 motion, which is originally common to all protoplasm, 

 is especially developed ; others effect sensation, which 

 power was possibly or probably present even in the 

 simple protoplasm. These will be fully discussed in the 

 following chapters. But before doing this, a few words 

 must be said as to one form of these modified cells, in 

 which the power of generating motion is already de- 

 veloped in a very noticeable degree, and serves partly 

 for the independent movement of the cell-body, or of 

 the animal of which the cell is a part ; partly, w^hen 

 occurring in fixed bodies, to move foreign matter — that 

 is, for the drawing in of food. 

 2 



