CHAP, xxv.] CONCLUDING REFLEXIONS. 569 



can be traced to a factor not clearly to be distinguished from 

 Symmetry itself. For, as has been explained, Symmetry, whether 

 Bilateral or Radial, is only a particular case of that phenomenon of 

 Repetition of Parts so universally characteristic of living bodies; 

 and that resemblance between two counterparts, which we call 

 Bilateral Symmetry, is akin to the resemblance between parts 

 repeated in Series, though, as is shewn by their geometrical re- 

 lations, the processes of division by which the parts were originally 

 set off, must be in some respects distinct. Bilateral Symmetry of 

 Variation is thus only a special case of the similar and simul- 

 taneous Variation of repeated parts. 



The greatness of the observed change from the normal is often 

 largely due to this possibility of simultaneity in Variation, the 

 change thus manifesting itself not in one part only, but in many 

 or all of the members of a series of repeated parts. Instances of 

 such similar and simultaneous Variation of serial parts in animals 

 have now been given. Examples still more marked may be seen 

 abundantly among plants. A variation, for example, in the form or 

 degree of fission of the leaf, slight perhaps by itself, when taken up 

 and repeated in every leaf in its degree, constitutes a definite and 

 conspicuous distinction. Everyone has observed this common fact. 

 Few illustrations of it are more evident than that of the common 

 Hawthorn. In a quickset hedge soon after the leaves begin to 

 unfold almost each separate plant can be recognized even at a 

 distance, and its branches can be traced by their special characters, 

 by the shapes and tints of the leaves, by the angles that they make 

 with the stem, by the manner of unfolding of the buds, and so 

 forth. These variations, sometimes slight in themselves, by their 

 similarity and simultaneity build up a conspicuous result. 



The phenomenon of serial resemblance is in fact an expression 

 of the capacity of repeated parts to vary similarly and simul- 

 taneously. In proportion as in their variations such parts retain 

 this capacity the relationship is preserved, and in proportion as it 

 is lost, and the parts begin to vary independently, exhibiting 

 differentiation, the relationship is set aside. It will be noticed 

 that to render the converse true we must extend the conception of 

 Serial Homology in special cases to organs not commonly regarded 

 as serially homologous with each other, but which having assumed 

 some common character thereafter may vary together (cp. p. 309). 



In the power of independent Variation, members of series once 

 more exhibit the property of "unity" that we have already noticed 

 as appearing in the manner in which the number of the members 

 is changed. The fact that members of series should be capable of 

 varying as " individuals " is paradoxical. Such members, teeth, 

 digits, segments of Arthropods, and the like, are each made up of 

 various tissues endowed with miscellaneous functions and dissimilar 

 in their morphological nature. Nevertheless each group is capable 



