94 THE STUDY OF ANIMAL LIFE CHAP. 



whole life of higher animals depends upon a coherence 

 of units. 



But how did this begin ? What of the gulf between 

 Protozoa and all the other animals which are many- 



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celled ? The gulf has been bridged, else we should not 

 exist ; but, more than that, part of the bridge is still 

 left. There are a few of the simplest animals which 

 form loose colonies of units, which, when they divide, re- 

 main together. Whether it was through some weakness 

 -hindering complete division that the transition forms 

 between Protozoa and higher animals became strong, w r e 

 do not know. All that w r e certainly know is that some 

 of the simplest animals form loose colonies of units, that 

 the gulf between them and the higher animals is thus 

 bridged, and that the bridging depends on coherence. 

 Our first conclusion, therefore, is, that the possibility 

 of there being any higher animals depends, primarily, 

 on the coherence of units. 



Our next step is this : When we study sponges, or 

 zoophytes, or most corals, or some types usually classed 

 as " worms," we see that the habit of forming colonies is 

 common. Every sponge is a simple sac to begin with, 

 but it buds off others like itself, and the result is a co- 

 herent colony. A zoophyte is not one individual, but a 

 connected colony of individuals. Throughout the colony 

 there is one life ; all the individuals have a common 

 origin, and all are members one of another. In varying 

 degrees of perfection the life of the whole is unified. 

 Moreover, the unity is often increased, not diminished, 

 by the fact that the individuals are not all alike. There 

 is division of labour among them. 



Our second conclusion is that among many animals- 

 beginning with sponges and ending with the sea-squirts, 

 which are acknowledged to be animals of high degree- 

 thc habit of forming colonies is common, and that these 

 colonies, though organically continuous, illustrate the 

 essence of society ; for in them many individuals of 

 common descent and nature are united in mutual depen- 

 dence and helpfulness. 



The next step towards an understanding of the social 



