234 MORPHOLOGICAL DEVELOPMENT. 



dence of forces tending ever to produce changed structural 

 arrangements. 



264. This broad statement of the correspondence be- 

 tween the general facts of Morphological Development and 

 the principles of Evolution at large, may be reduced to 

 statements of a much more specific kind. The phenomena 

 of symmetry and unsymmetry and asymmetry, which we 

 have traced out among organic forms, are demonstrably in 

 harmony with those laws of the re-distribution of matter and 

 motion to which Evolution conforms. Besides the myriad- 

 fold illustrations of the instability of the homogeneous, 

 afforded by these aggregates of units of each order, which, 

 at first alike, lapse gradually into unlikeness; and besides 

 the myriad-fold illustrations of the multiplication of effects, 

 which these ever-complicating differentiations exhibit to us; 

 we have also myriad-fold illustrations of the definite equali- 

 ties and inequalities of structures, produced by definite equali- 

 ties and inequalities of forces. 



The proposition arrived at when dealing with the causes 

 of Evolution, " that in the actions and reactions of force and 

 matter, an unlikeness in either of the factors necessitates an 

 unlikeness in the effects; and that in the absence of unlike- 

 ness in either of the factors the effects must be alike " (First 

 Principles, 169), is a proposition which implies all these 

 particular likenesses and unlikenesses of parts which we 

 have been tracing. For have we not everywhere seen that 

 the strongest contrasts are between the parts that are most 

 contrasted in their conditions ; while the most similar parts 

 are those most-similarly conditioned? In every plant the 

 leading difference is between the attached end and the free 

 end; in every branch it is the same; in every leaf it is 

 the same. And in every plant the leading likenesses are 

 those between the two sides of the branch, the two sides of 

 the leaf, and the two sides of the flower, where these parts 

 are two-sided in their conditions ; or between all sides of the 



