112 THE THEORIES OF EVOLUTION 



up of molecules, and clusters of them make up the 

 cells. The shape of the organism depends upon the 

 shape of the cells; Spencer holds that there are as 

 many categories of these elementary units as there are 

 species of living things, as each category produces one 

 certain organic form. In order to make the process 

 more readily understood, Spencer compares the phys- 

 iological units to the chemical molecules of a crystal- 

 loid body. Like chemical molecules, which always 

 group themselves so as to produce one definite shape, 

 cube, prism, or rhombohedron, the physiological units 

 group themselves so as to constitute an organism 

 whose shape, however complicated it may appear, is 

 invariably the same for all the individuals of one sin- 

 gle species. 



Under the influence of polarity, every individual 

 has to assume the shape of the species to which it be- 

 longs. A bird must have feathers, a beak and cer- 

 tain internal organs, just as a certain substance when 

 crystallised must present facets inclining to each other 

 at definite angles and invariably reproducing one and 

 the same form. 



And yet there is, as Spencer points out, a certain 

 difference : The aggregation of units constituting a 

 living organism is more complex than any combina- 

 tion of chemical molecules; it is therefore less stable, 

 and its equilibrium can be more easily disturbed by the 

 pressure of various incidental forces. The shape pro- 



