THE GENERAL SHAPES OF ANIMALS. 189 



remarkable illustrations of the way in wliicli forms become 

 moulded into harmony with incident forces. For besides 

 tliis divergence from bilateral sjnnmetry involved by the 

 presence of both eyes upon the upper side, there is a further 

 divergence from bilateral symmetry involved by the differ- 

 entiation of the two sides in respect to the contours of their 

 surfaces and the sizes of their fins. And then, what is still 

 more significant, there is a near approach to likeness be- 

 tween the halves that were originally unlike, but are, under 

 the new circumstances, exposed to lil^e conditions. The 

 body is divisible into similarly- shaped parts by a plane 

 cuttino: it alono^ the side from head to tail: " the dorsal and 

 ventral instead of the lateral halves become symmetrical in 

 outline and are equipoised." 



§ 253. Thus, little as there seems in common between the 

 shapes of plants and the shapes of animals, we yet find, on 

 analysis, that the same general truths are displayed by 

 both. The one ultimate principle that in any organism 

 equal amounts of growth take place in those directions in 

 which the incident forces are equal, serves as a key to the 

 phenomena of morphological differentiation. By it we are 

 furnished ■v\dth intarpretations of those likenesses and un- 

 likenesses of parts, which are exhibited in the several kinds 

 of s}Tnmetry; and when we take into account inherited 

 effects, wrought under ancestral conditions contrasted in 

 various ways with present conditions, we are enabled to 

 comprehend, in a general way, the actions by which animals 

 have been moulded into the shapes they possess. 



To fill up the outline of the argument, so as to make it 

 correspond througliout with the argimient respecting vegetal 

 forms, it would be proper here to devote a chapter to the 

 differentiations of those homologous segments out of which 

 animals of certain types are composed. Though, among most 

 animals of the third degree of composition, such as the root- 

 ed Hydrozoa, the Polyzoa, and the Ascidioida, the united 



