THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 93 



ciently clear, that the compound Coslenterata are aggregates 

 of the third order, produced by integration of aggregates of 

 the second order such as we have in the Hydra. As before, 

 it is manifest that on the hypothesis of evolution, these 

 higher integrations will insensibly arise, if the separation of 

 the ge nnniparous polypes is longer and longer postponed; 

 and that an increasing postponement will result by survival 

 of the fittest, if it profits the group of individuals to remain 

 united instead of dispersing.* 



§ 203. The like relations exist, and imply that the like 

 processes have been gone through, among those more highly- 

 organized animals called Polyzoa and Tunicata. We have 

 solitary individuals, and we have variously-integrated groups 

 of individuals: the chief difference between the evidence 

 here furnished, and that furnished in the last case, being the 

 absence of a type obviously linking the solitary state with 

 the aggregated state. 



This integration of aggregates of the second order, is car- 

 ried on among the Polyzoa in divers ways, and with different 

 degrees of completeness. The little patches of minute cells, 

 shown as magnified in Fig. 153, so common on the fronds of 

 sea-weeds and the surfaces of rocks at low- water mark, display 

 little beyond mechanical combination. The adjacent indi- 



154: 



* A critic says the question is u what are the forces internal or external 

 which produce union or separation." A proximate reply is — degree of nutri. 

 tion. As in a plant new individuals or rudiments of them are cast off where 

 nutrition is failing, so in a compound animal. The connecting part dwindles 

 if it ceases to carry nutriment. 





