204 THE INDUCTIONS OF BIOLOGY. 



them the general duty of expending energy, 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 separa- 

 tion of the osseous system, which has to resist the strains of 

 muscular action, from the muscular system, which, in gene- 

 rating motion, entails these strains. Simultaneously there 

 have been going on similar actual and potential specializa- 

 tions in the functions of accumulating energy and transfer- 

 ring energy. And throughout 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 physiological 

 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 govern- 

 ing class and the governed class; how while in the govern- 

 ing class there slowly grow up such differences of duty as the 

 civil, military, and ecclesiastical, there arise in the governed 

 class fundamental industrial differences like those between 

 agriculturists and artizans; and how there is a continual 

 multiplication of such specialized occupations and special- 

 ized shares of each occupation. 



59. Fully to understand this change from homogeneity 

 of function to heterogeneity of function, which accompanies 

 the change from homogeneity of structure to heterogeneity of 

 structure, it is needful to contemplate it under a converse 



