§ 176. The problems of Morphology fall into two distinct classes, answering respectively to the two leading aspects of Evolution. In things which evolve there go on two processes—increase of mass and increase of structure. Increase of mass is primary, and in simple evolution takes place almost alone. Increase of structure is secondary, accompanying or following increase of mass with more or less regularity, wherever evolution rises above that form which small inorganic bodies, such as crystals, present to us. As the fundamental antagonism between Dissolution and Evolution consists in this, that while the one is an integration of motion and disintegration of matter, the other is an integration of matter and disintegration of motion; and as this integration of matter accompanying disintegration of motion, is a necessary antecedent to the differentiation of the matter so integrated; it follows that questions concerning the mode in which the parts are united into a whole, must be dealt with before questions concerning the mode in which these parts become modified.[1]
This is not obviously a morphological question. But an illustration or two will make it manifest that fundamental differences may be produced between aggregates by differences in the degrees of composition of the increments: the ultimate units of the increments being the same. Thus an accumulation of things of a given kind may be made by adding one at a time. Or the things may be tied up into bundles of ten, and the tens placed together. Or the tens may be united into hundreds, and a pile of hundreds formed. Such unlikenesses in the structures of masses are habitually seen in our mercantile transactions. Articles which the consumer recognizes as single, the retailer keeps wrapped up in dozens, the wholesaler sends in gross, and the manufacturer supplies in packages of a hundred gross. That is, they severally increase their stocks by units of simple, of compound, and of doubly-compound kinds. Similarly result those differences of morphological composition which we have first to consider. An organism consists of units. These units may be aggregated into a mass by the addition of unit to unit. Or they may be united into groups, and the groups joined together. Or these groups of groups may be so combined as to form a doubly-compound aggregate. Hence there arises respecting each organic form the question—is its composition of the first, second, third, or fourth order?—does it exhibit units of a singly-compounded kind only, or are these consolidated into units of a doubly-compounded kind, or a triply-compounded kind? And if it displays double or triple composition, the homologies of its different parts become problems. Under the disguises induced by the consolidation of primary, secondary, and tertiary units, it has to be ascertained which answer to which, in their degrees of composition.
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?
Very numerous are the heterogeneities of form presenting themselves for interpretation under these heads. The ultimate morphological units combined in any group, may be differentiated 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 multiform 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, numerous 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 phenomena the process of evolution; and to show how, as displayed in them, it conforms to those first principles which evolution in general conforms to. Two sets of factors have to be taken into account. Let us look at them.
The factors of the first class are those which tend directly to change an organic aggregate, in common with every other aggregate, from that more simple form which is not in equilibrium with incident forces, to that more complex form which is in equilibrium with them. We have to mark how, in correspondence with the universal law that the uniform lapses into the multiform, and the less multiform into the more multiform, the parts of each organism are ever becoming further differentiated; and we have to trace the varying relations to incident forces by which further differentiations are entailed. We have to observe, too, how each primary modification of structure, induced by an altered distribution of forces, becomes a parent of secondary modifications—how, through the necessary multiplication of effects, change of form in one part brings about changes of form in other parts. And then we have also to note the metamorphoses constantly being induced by the process of segregation—by the gradual union of like parts exposed to like forces, and the gradual separation of like parts exposed to unlike forces. The factors of the second class which we have to keep in view throughout our interpretations, are the formative tendencies of organisms themselves—the proclivities inherited by them from antecedent organisms, and which past processes of evolution have bequeathed. We have seen it to be inferable from various orders of facts (§§ [65], [84], [97–97g]), that organisms are built up of certain highly-complex molecules, which we distinguished as physiological units [or constitutional units as they might otherwise be called]—each kind of organism being built up of units peculiar to itself. We recognized in these units, powers of arranging themselves into the forms of the organisms to which they belong; analogous to the powers which the molecules of inorganic substances have of aggregating into specific crystalline forms. We have consequently to regard this proclivity of the physiological units, as producing, during the development of any organism, a combination of internal forces that expend themselves in working out a structure in equilibrium with the forces to which ancestral organisms were exposed; but not in equilibrium with the forces to which the existing organism is exposed, if the environment has been changed. Hence the problem in all cases is, to ascertain the resultant of internal organizing forces, tending to reproduce the ancestral form, and external modifying forces, tending to cause deviations from that form. Moreover, we have to take into account, not only the characters of immediately-preceding ancestors, but also those of their ancestors, and ancestors of all degrees of remoteness. Setting out with rudimentary types, we have to consider how, in each successive stage of evolution, the structures acquired during previous stages have been obscured by further integrations and further differentiations; or, conversely, how the lineaments of primitive organisms have all along continued to manifest themselves under the superposed modifications.
§179. Two ways of carrying on the inquiry suggest themselves. We may go through the several great groups of organisms, with the view of reaching, by comparison of parts, certain general truths respecting the homologies, the forms, and the relations of their parts; and then, having dealt with the phenomena inductively, may retrace our steps with the view of deductively interpreting the general truths reached. Or, instead of thus separating the two investigations, we may carry them on hand in hand—first establishing each general truth empirically, and then proceeding to the rationale of it. This last method will, I think, conduce to both brevity and clearness. Let us now thus deal with the first class of morphological problems.
[Note.—In preparation for treating of morphological development, sundry other general considerations should have been included in the foregoing chapter when originally published. This seems the most appropriate place for now naming them. Some were implicitly contained in the first volume, but it will be well definitely to state these, as well as the others not yet implied.