8IMILAEITY OF GROUPINGS. 391 



A further series of changes begins now to take place ; to 

 <vhich, as before, we shall find analogies in individual or- 

 ganisms. Returning again to the Hydrozoa^ we observe 

 that in the simplest of the compound forms, the connected 

 individuals developed from a common stock, are alike in 

 structure, and perform like functions : with the exception, 

 indeed, that here and there a bud, instead of developing 

 into a stomach, mouth, and tentacles, becomes an egg-sac. 

 But w^ith the oceanic Hydrozoa^ this is by no means the 

 case. In the Calycophoridoe^ some of the polypes growing 

 from the common germ, become developed and modified 

 into large, long, sack-like bodies, which by their rhythmi- 

 cal contractions move through the water, dragging the 

 community of polypes after them. In the Physophoridae., 

 a variety of organs similarly arise by transformation of the 

 budding polypes ; so that in creatures like the Physalia^ 

 commonly known as the " Portuguese Man-of-war," instead 

 of that tree-like group of similar individuals forming the 

 original type of the class, we have a complex mass of unlike 

 parts fulfilling unlike duties. As an individual Hydra may 

 be regarded as a group of Protozoa^ which have become 

 partially metamorphosed into difierent organs ; so a Phy- 

 salia is, morphologically considered, a grou^) of HijdrcB of 

 which the individuals have been variously transformed to 

 fit them for various functions. 



This differentiation upon differentiation, is just what 

 takes place in the evolution of a civilized society. We ob- 

 served how, in the small communities first formed, there 

 arises a certain simple political organization — there is a 

 partial separation of classes having different duties. And 

 now we have to observe how, in a nation formed by the 

 fusion of such small communities, the several sections, at 

 first alike in structures and modes of activity, gradually 

 become unlike in both — gradually become mutually-de- 

 pendent parts, diverse in their natures and functions. 



