116 MORPHOLOGICAL DEVELOPMENT. 



tions that result from increase of mass and increase of com- 

 position. In each part considered individuaUy, there arises 

 a characteristic shape consequent on that relative position 

 towards external and internal forces, which the mode of 

 growth entails. Every member of the aggregate presents 

 itseKin a more or less peculiar way towards the light, towards 

 the air, and towards its point of support ; and according to 

 the relative homogeneity or heterogeneity in the incidence of 

 the agencies thus brought to bear on it, wiU be the relative 

 homogeneity or heterogeneity of its shape. 



§ 216. Before passing from this a priori view of the mor- 

 phological differentiations which necessarily accompany 

 morphological integrations, to an a posteriori view of them, it 

 seems needful to specify the meanings of certain descriptive 

 terms we shall have to employ. 



Taking for our broadest division among forms, the regular 

 and the irregular, we may divide the latter into those which 

 are wholly irregular and those which, being but partially 

 irregular, suggest some regular form to which they approach. 

 By slightly straining the difference between them, two current 

 words may be conveniently used to describe these subdivi- 

 sions. The entirely irregular forms we may class as 

 asymmetrical — ^literally as forms without any equality of 

 dimensions. The forms which approximate towards regu- 

 larity without reaching it, we may distinguish as imsym- 

 metrical — a word which, though it asserts inequality of 

 dimensions, has been associated by use rather with such 

 slight inequality as constitutes an observable departure from 

 equality. 



Of the regular forms there are several classes, differmg in 

 the number of directions in which equality of dimensions is 

 repeated. Hence results the need for names by which sym- 

 metry of several kinds may be expressed. 



The iQost regular of figures is the sphere : its dimensions 

 are the same from centre to surface in all directions ; and ii 



