160 THE INDUCTIONS OF BIOLOGY. 



rudiment of the nervous system. This indication of struc- 

 tures which are to share between them the general duty of 

 expending force, is soon followed by changes that foreshadow 

 further specializations of this general duty. In the incipient 

 nervous system, there begins to arise that contrast between 

 the cerebral mass and the spinal cord, which, in the main, 

 answers to the division of nervous actions into directive 

 and executive ; and at the same time, the appearance of 

 vertebral laminae foreshadows the separation of the osseous 

 system, which has to resist the strains of muscular action, 

 from the muscular system, which, in generating motion, en- 

 tails these strains. Simultaneously there have been going 

 on similar actual and potential specializations in the functions 

 of accumulating force and transferring force. And through- 

 out all subsequent phases, the method is substantially the 

 same. 



This progress from general, indefinite, and simple kinds 

 of action, to special, definite, and complex kinds of action, 

 has been aptly termed by Milne-Edwards, the " physio- 

 logical division of labour." Perhaps no metaphor can more 

 truly express the nature of this advance from vital activity 

 in its lowest forms to vital activity in its highest forms. 

 And probably the general reader cannot in any other way 

 obtain so clear a conception of functional development in 

 organisms, as he can by tracing out functional development in 

 societies : noting how there first comes a distinction between 

 the governing class and the governed class ; how while in 

 the governing class there slowly grow up such differences of 

 duty as the civil, military, and ecclesiastical, there arise in 

 the governed class, fundamentally industrial differences like 

 those between agriculturists and artizans ; and how there is 

 a continual multiplication of such specialized occupations, 

 and specialized shares of each occupation. 



§ 59. Fully to understand this change from homogeneity 

 to heterogeneity of function, which accompanies the change 



