CHAPTER XIII. 



MORPHOLOGICAL DIFFERENTIATION IN ANIMALS. 



§ 242. The general considerations which preluded our in- 

 quiry into the shapes of plants and their parts, equally serve, 

 so far as they go, to prelude an inquiry into the shapes of 

 animals and their parts. Among animals, as among plants, 

 the formation of aggregates greater in bulk or higher in 

 degree of composition, or both, is accompanied by changes of 

 form in the aggregates as wholes as well as by changes of 

 form in their parts ; and the processes of morphological 

 differentiation conform to the same general laws in the one 

 kingdom as in the other. 



It is needless to recapitulate the several kinds of modifi- 

 cation to be explained, and the several factors that co- 

 operate in working them. In so far as these are common 

 to plants and animals, the preceding chapters have suf- 

 ficiently familiarized them. Nor is it needful to specify 

 afresh the several types of symmetry and their descriptive 

 names ; for what is true of them in the one case is true of 

 them in the other. There is, however, one new and all- 

 important factor which we shall have now to take into 

 account ; and about this a few preliminary remarks are 

 requisite. 



§ 243. This new factor is motion — motion of the organism 

 in relation to surrounding objects, or of the parts of the 



