THE GENERAL SHAPES OP ANIMALS. 191 



lish themselves. Such deviations must tend to destroy the 

 original indefiniteness and variability of attitude — must cause 

 gravitation towards an habitual attitude. And gravitation 

 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 render- 

 ing more decided the actions that conduce to bilateralness. 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 

 * Criticisms on the above passage have shown the need for naming sun- 

 dry complications. These complications chiefly, if not wholly, arise from 

 changes in modes of life — changes from the locomotive to the stationary, and 

 from the stationary to the locomotive. Referring to my statement that (ignor- 

 ing the spherical) the radial type is the lowest and must be taken as ante- 

 cedent to the bilateral type, it is alleged that all existing "radial animals 

 above Protozoa are probably derived from free swimming, bilaterally-sym- 

 metrical animals." If this is intended to include the planulse of the hydroid 

 polyps, then it seems rather a straining of the evidence. These locomotive 

 embryos, described as severally having the structure of a gastrula with a 

 closed mouth, can be said to show bilateralness only because the first two ten- 

 tacle^ make their appearance on opposite sides of the mouth — a bilateralness 

 which lasts only till two other tentacles make their appearance in a plane at 

 right angles, so giving the radial structure. I think the criticism applies only 

 to cases furnished by Echinoderms. The larvae of these creatures have bilat- 

 erally-symmetrical structures, which they retain as long as they swim about 

 and which such of them as fix themselves lose by becoming similarly related 

 to conditions all round : the radial structure being retained by those types 

 which, becoming subsequently detached, move about miscellaneously. But, as 

 happens in some of the Sea-urchins and still more among the Holothurians, 

 the structure is again made bilaterally-symmetrical by a locomotive life pur- 

 sued with one end foremost. Should it be contended that the conditions and 

 the forms are reciprocally influential — that either may initiate the other, it 

 still remains unquestionable that ordinarily the conditions are the antecedents, 

 as is so abundantly shown by plants. 



