EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 297 



together produced units as thoroughly distinct as those of the sponge, 

 but nevertheless connected in the work of nourishing and repairing 

 the colony. In the " sea-mats " (Fig. 190 ) we see a stage of colonial 

 development in an animal form which more nearly approaches 

 the condition of the higher animals, but which likewise lacks all 

 the intimate features of connected interests seen therein. The 

 " sea-mat " colony is an aggregate of units each of which we have 

 seen to be perfectly independent, save for external connections, of 

 its neighbour units. There must thus exist a certain and not 

 distant parallelism between a "sea-mat's" constitution and that of 

 higher beings ; inasmuch as both are colonial, and in both the 

 units exist in a relative but by no means corresponding degree of 

 independence. 



Analogies are thus plentiful enough in showing us the stages which 

 intervene between the dependence and connection of the units in 

 higher life, and the comparative independence of those in lower life. 

 But the cases of the Nai's or river- worm (Fig. 192), as well as those of 

 the plant-lice and bees, show us plainly enough the amazing possibi- 

 lities of highly organised animals becoming " colonial " organisms, 

 even with complete separation and detachment of the units of the 

 colony which, however, in the case of the bees, as "social" 

 insects, is again reconstructed in the institution of a co-operative 

 life and existence. In the Nai's, we see illustrated a tendency 

 towards repetition of "zooids," which may be viewed as leading 

 towards an appreciation of the manner in which an originally 

 jointed animal itself colonial in one sense advances towards the 

 condition of the plant-lice and bees with free and separate units. 

 It is not more surprising, we may repeat, to find the insect- 

 individual with its separated and detached units, than to discover 

 in the higher bird or quadruped the same colonial structure, but 

 one likewise which is closely combined and intimately related 

 as to its elementary parts. The possibilities of life, are facts, indeed, 

 which in the present case cut both ways; demonstrating, even if 

 leaving the main collateral facts unexplained, how in the higher 

 spheres of animal society, the independence of an animal colony 

 may perfectly co-exist with the interdependence of its original units. 



But there exists for the biologist a final and authoritative court 

 of appeal in the matter of the origin of the colonial constitution and 

 its modifications, in the facts and teachings of development. The 

 general tendency of any organism undergoing development is, as we 

 have seen, one leading it towards differentiation and division of its 

 primitive and originally simple substance. Even in the lowest confines 

 of life we witness this tendency towards segregation and multiplica- 

 tion of its parts. The gregarina (Fig. 183, 'a) exhibits such a process, 

 and the early stages of all living beings are marked by the segmentation 



