206 MORPHOLOGICAL DEVELOPMENT. 



along with a general twist. This metamorphosis furnishes 

 several remarkable illustrations of the way in which forms 

 become moulded into harmony with incident forces. For 

 besides the divergence from bilateral symmetry involved by 

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

 divergence from bilateral symmetry involved by differentiation 

 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 between the 

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

 circumstances, exposed to like conditions. The body is 

 divisible into similarly-shaped parts by a plane cutting it 

 along 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 

 with interpretations of those likenesses and unlikenesses of 

 parts, which are exhibited in the several kinds of symmetry; 

 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 gen- 

 eral 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 throughout with the argument 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 



