166 THE EVOLUTION OF CONTINUITY 



of the formation of horny periderm which defeated the 

 forces working for zooidal discontinuity ; though the for- 

 mation itself might well be a result of attachment and 

 immobility connected with questions of relative density. As 

 regards the Discontinuous Megazooid of anemonoid form 

 in a preceding chapter its structural plan has been shown 

 to be attributable to the compression of potential branching 

 zooidal plan during development (pages 87-90), and it need 

 only be remarked that our theory is that water-pressure was 

 the factor at work : it being presumed that a fertilised 

 ovum with continuously zooidal potentialities came to 

 develop in heavier pressure conditions than those which 

 evolved the ancestral type. 



The medusiform gonophore, which appears suddenly 

 after a considerable amount of continuously zooidal growth 

 and not directly from a fertilised ovum, is more difficult to 

 explain. Yet it is clearly a spray of sporosacs which has been 

 compressed in its development to form a sexual megazooid, 

 and it is reasonable to suppose that its formation would 

 indicate a special compressibility, perhaps related to an 

 increasing density and a plasticity connected with approach- 

 ing cycle-termination. In the Continuously Megazooidal 

 Corals, on the other hand, where the coralline plan is that of 

 the anemone, the continuity displayed is probably the result 

 of megazooidal density and attachment, which in turn 

 were factors in the deposition of the coralline skeleton 

 binding all the megazooids together. 



The manner in which water-pressure could have evolved 

 Segmental Continuity will be mentioned presently in con- 

 nection with the evolution of the primitive Fish ; active 

 movement in the presence of water-pressure being in this 

 case a factor of special importance. 



But it has been taken for granted that some factor 

 intervened to place fertilised ova, or conjugated gametes, 

 in conditions where water-pressure greater than the habitual 

 obtained. And we have this factor in Segregation. 



Segregation acts in many guises, but it has been instru- 

 mental in the origination not only of the different types 

 of Continuity, but also of the different species within each 

 type. 



