222 THE EVOLUTION OF CONTINUITY 



we may say that the motility displayed by so many living 

 organisms, in spite of any opposition which Gravity and 

 Solar attraction can offer, is proof that Food-attraction can 

 defeat both these forces. Naturally, Food-attraction, though 

 probably the most powerful of factors calling for motility, 

 is only one of many ; and sources of repulsive force could 

 demand movement away from their neighbourhood. 



As we shall see, there is a Symmetry associated with 

 immobility, and a Symmetry associated with motility, and 

 they are distinct. 



It is not the purpose here to take one by one the 

 different types of Individual Continuity in our brief survey. 

 Certain of these can be ruled out as asymmetrical ; for 

 as Individuals they develop as colonies, growing in serial 

 and lateral extension, and branching irregularly. Thus we 

 exclude the Filamentous, Ccenocytic, Continuously Zooidal, 

 and Continuously Megazooidal Individuals. And such as 

 the Discontinuously Zooidal, Megazooidal, and Segmental 

 can have no symmetry of form, owing to their component 

 units being free and independent. The problem, in fact, 

 narrows itself down to the Symmetry of certain organisms 

 which may, as in the distinct zooid, be part of an Individual, 

 or as in the higher segmental organism be the whole 

 Individual. It is the problem of the symmetry of the zooid, 

 the megazooid, and the segmental organism. It is to the 

 last of these, and its bilateral symmetry, that most attention 

 shall be given. 



We may credit a zooid, such as the Hydra zooid, with 

 being built on a symmetrical plan, though the contractile 

 powers of the organism tend to obscure it. For all purposes 

 it has the symmetry of a cylinder. It is impossible to say 

 definitely how this cylindrical form had to take shape, 

 though on page 164 we have made a few speculations on 

 the matter ; but we would attribute the tubular form of 

 the zooid to the effects of water-pressure on the development 

 of a zygote whose inherited potentialities were filamentous. 

 So that an inherited power of developing as discrete branching 

 series of cells was realised as lines of cells united in lateral 

 continuity. It is not difficult to see how even water-pressure 

 acting on all sides could give and maintain a cylindrical 



