Such questions are more intricate than they at first appear; since, besides the obscurities caused by progressive integration, and those due to accompanying modifications of form, further obscurities result from the variable growths of units of the different orders. Just as an army may be augmented by recruiting each company, without increasing the number of companies; or may be augmented by making up the full complement of companies in each regiment, while the number of regiments remains the same; or may be augmented by putting more regiments into each division, other things being unchanged; or may be augmented by adding to the number of its divisions without altering the components of each division; or may be augmented by two or three of these processes at once; so, in organisms, increase of mass may result from additions of units of the first order, or those of the second order, or those of still higher orders; or it may be due to simultaneous additions to units of several orders. And this last mode of integration being the general mode, puts difficulties in the way of analysis. Just as the structure of an army would be made less easy to understand if companies often outgrew regiments, or regiments became larger than brigades; so these questions of morphological composition are complicated by the indeterminate sizes of the units of each kind: relatively-simple units frequently becoming more bulky than relatively-compound units. § 177. The morphological problems of the second class are those having for their subject-matter the changes of shape which accompany changes of aggregation. The most general questions respecting the structure of an organism, having been answered when it is ascertained of what units it is composed as a whole, and in its several parts; there come the more 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 specifically--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 proportions 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?