i?4 MORPHOLOGICAL DEVELOPMENT. 



cause gravitation towards an habitual attitude. And gravita- 

 tion towards an habitual attitude having once commenced, 

 will continually increase, where increase of it is not negatived 

 by adverse agencies : each further degree of bilateralness 

 rendering more decided the actions that conduce to bilateral- 

 ness. If this reply be thought insufficient, it may be enforced 

 by the further one, that as, among plants, the incident forces 

 are the antecedents and the forms the consequents (changes of 

 forces being in many cases visibly followed by changes of 

 forms) we are warranted in concluding that the like order of 

 cause and effect holds among animals. 



247. Keeping to the same type but passing to a higher 

 degree of composition, we meet more complex and varied 

 illustrations of the same general laws. In the compound 

 Ccelenterata, presenting clusters of individuals that are 

 severally homologous with the solitary individuals last dealt 

 with, we have to note both the shapes of the individuals thus 

 united, and the shapes of the aggregates made up of them. 



Such of the fixed Hydrozoa and Actinozoa as form branched 

 societies, continue radial ; both because their varied attitudes 

 do not expose them to appreciable differences in their rela- 

 tions to those surrounding actions which chiefly concern 

 them (the actions of prey), and because such differences, even 

 if they were appreciable, would be so averaged in their 

 effects on the dissimilarly-placed members of each group as 

 to be neutralized in the race. Among the 

 tree-like coral-polypedoms, as well as in 

 such ramified assemblages of simpler poly- 

 pes as are shown in Figs. 149, 150, we 

 have, indeed, cases in many respects paral- 

 lel to the cases of scattered flowers ( 233), 

 which though placed laterally remain radial, 

 because no differentiating agency can act 

 uniformly on all of them. Meanwhile, in the groups 



which these united individuals compose, we see the shapes of 



