SPECIALIZATION AND ORGANIZATION. II 



by the contending parties is utterly irreconcilable, still 

 we can understand how, by modifying our potentialities 

 in one direction and our actualities in another, the dif- 

 ference might be brought near a vanishing point. 



Whether we look at the successive stages connecting 

 the relatively homogeneous germ with the completed 

 organism, or at the paleontological succession of forms, 

 we see that progress in the organic world is always from 

 the less to the more heterogeneous. As division of labor 

 advances, complexity of structure increases, and the ties 

 of mutual dependence multiply and strengthen. In a 

 word, the most characteristic trait of evolution is, that 

 increasing division of labor conditions increasing union 

 of the laborers. Division and union, differentiation and 

 integration, specialization and organization, march hand 

 in hand. 



The same truth comes perhaps more clearly into view, 

 when, taking the protozoan colony for our starting-point, 

 we run up the scale of animal organizations. Passing 

 on from the Volvox colony, we soon come to an instruc- 

 tive stage represented in the common fresh-water Hydra. 

 Although we now know that the organization of this 

 animal is far from being as simple as was supposed 

 by its discoverer, Trembley, and by other naturalists of 

 his time, who regarded it as a connecting link between 

 plants and animals, still it affords a striking illustration 

 of the fact, that physiological nnity is a thi^ig of degrees^ 

 inco7nplete according as the division of labor is low. 



A single division of labor, in advance of what we saw 

 in Volvox, makes Hydra an unmistakable Metazoon, plac- 

 ing it fairly on the main line of animal evolution. It is 

 the separation of the digestive from the other functions 



