296 CHAPTERS ON EVOLUTION. 



" colonial " aggregates of cells, than to discover that certain of 

 the cells thereof have developed an independence and freedom of 

 motion equal to that of an animalcule living in its native haunts, and 

 carried out through movements of exactly similar nature to those 

 performed by the amoeba itself. Thus a first halting-place in our 

 philosophy of individuality may readily be found in the declaration 

 that the " colonial " or " compound " body is in reality the normal 

 constitution of all animals, save the very lowest With the advance 

 of life there has been exhibited a progress in complexity, and this 

 progress has found structural expression in the conversion and 

 multiplication of the original unit of the germ into the colonial and 

 compound state. We ourselves are "compound" individuals, in 

 the sense that our physical personality is not single in any sense, 

 but markedly multiple. Our individuality may be named doubly 

 " compound," in the sense that, whilst each tissue may be held to 

 represent a " zooid," or colonial member of the body as a whole, the 

 tissues are, in their turn, made up of " cells," each of which is a 

 distinct morphological unit. 



If the above deduction be correct, founded as it is upon strict 

 anatomical detail, it remains to discuss those cases of " colonial 

 aggregations " in which the separate units are plainly recognisable, 

 as in zoophyte, sea-mat, and tapeworm. Such cases will be found 

 to differ not in kind, but in degree only, from the higher colonial 

 organisations we have just described. The zoophyte and the highest 

 animal are separated by a gulf not impassable or fixed, when the aid of 

 broad generalisation in comparative anatomy is invoked. For there are, 

 firstly, gradations and stepping-stones connecting the two extremes ; 

 and there exists, moreover, a general principle of development whereby 

 the differences between the colonial nature of the higher and that of 

 the lower form may be aptly expressed. Thus the sponge illustrates 

 a case in which the colonial nature of the highest organisms is plainly 

 enough foreshadowed. A sponge or a hydra advances but a tithe of 

 the developmental journey which a bird or quadruped has to pursue ; 

 and as a result of its early arrest on the developmental pathway, its 

 component units evince but little elaboration on their primitive and 

 animalcular state. If a sponge is a mass of amoebae, as to its living 

 parts, it exists in this simple condition because there was no further 

 need for a more intimate relationship between its various units. The 

 fact, already mentioned, that two fresh-water sponges, placed in con- 

 tact, unite into one, shows the ill-defined nature of the individuality 

 in a case like the present, where the units are merely placed in appo- 

 sition, so to speak, and united simply by the common skeleton they 

 elaborate. In a zoophyte (Fig. 189), which is in reality but little 

 removed above the sponge in the animal series, development and its 

 attendant conditions whatever these latter may have been have 



