THE PROBLEMS OF MORPHOLOGY. 7 



special questions concerning its form form in the ordinary 

 sense. After the contrasts caused by variations in the process 

 of integration, we have to consider the contrasts caused by 

 variations in the process of differentiation. To speak speci- 

 fically the shape of the organism as a whole, irrespective 

 of its composition, has to be accounted for. Reasons have 

 to be found for the unlikeness between its general outlines 

 and the general outlines of allied organisms. And there 

 have to be answered kindred inquiries respecting the propor- 

 tions of its component parts: Why, among such of these as 

 are homologous with one another, have there arisen the 

 differences that exist? And how have there been produced 

 the contrasts between them and the homologous parts of 

 organisms of the same type ? 



Very numerous are the heterogeneities of form presenting 

 themselves for interpretation under these heads. The ulti- 

 mate morphological units combined in any group, may be dif- 

 ferentiated individually, or collectively, or both : each of them 

 may undergo changes of shape; or some of them may be 

 changed and others not; or the group may be rendered mul- 

 tiform by the greater growth of some of its units than of 

 others. Similarly with the compound units arising by union 

 of these simple units. Aggregates of the second order may 

 be made relatively complex in form, by inequalities in the 

 rates of multiplication of their component units in diverse 

 directions; and among a number of such aggregates, numer- 

 ous unlikenesses may be constituted by differences in their 

 degrees of growth, and by differences in their modes of growth. 

 Manifestly, at each higher stage of composition the possible 

 sources of divergence are multiplied still further. 



That facts of this order can be accounted for in detail is 

 not to be expected the data are wanting. All that we may 

 hope to do is to ascertain their general laws. How this is to 

 be attempted we will now consider. 



178. The task before us is to trace throughout these 



