216 MORPHOLOGICAL DEVELOPMENT. 



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 state- 

 ments 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 har- 

 mony 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, that 

 are 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 equal- 

 ities and inequalities of structures, produced by definite 

 equalities 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, 129), is the general formula including 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 

 branch, all sides of the leaf, and all sides of the flower, where 

 these parts are similarly conditioned on all sides. So, too, is 

 it with animals that move about. The most marked contrasts 



