It is, indeed, a wholly gratuitous assumption that organisms admit of being placed in groups of equivalent values; and that these may be united into larger groups which are also of equivalent values; and so on. There is no à priori reason for expecting this; and there is no à posteriori evidence implying it, save that which begs the question—that which asserts one distinction to be generic and another to be ordinal, because it is assumed that such distinctions must be either generic or ordinal. The endeavour to thrust plants and animals into these definite partitions is of the same nature as the endeavour to thrust them into linear series. Not that it does violence to the facts in anything like the same degree; but still, it does violence to the facts. Doubtless the making of divisions and sub-divisions, is extremely useful; or rather, it is necessary. Doubtless, too, in reducing the facts to something like order they must be partially distorted. So long as the distorted form is not mistaken for the actual form, no harm results. But it is needful for us to remember that while our successively subordinate groups have a certain general correspondence with the realities, they tacitly ascribe to the realities a regularity which does not exist.
§ 102. A general truth of much significance is exhibited in these classifications. On observing the natures of the attributes which are common to the members of any group of the first, second, third, or fourth rank, we see that groups of the widest generality are based on characters of the greatest importance, physiologically considered; and that the characters of the successively-subordinate groups, are characters of successively-subordinate importance. The structural peculiarity in which all members of one sub-kingdom differ from all members of another sub-kingdom, is a peculiarity that affects the vital actions more profoundly than does the structural peculiarity which distinguishes all members of one class from all members of another class. Let us look at a few cases.
We saw ([§ 56]), that the broadest division among the functions is the division into "the accumulation of energy (latent in food); the expenditure of energy (latent in the tissues and certain matters absorbed by them); and the transfer of energy (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." Now in the lowest animals, united under the general name Protozoa, there is either no separation of the parts performing these functions or very indistinct separation: in the Rhizopoda, all parts are alike accumulators of energy, expenders of energy and transferers of energy; and though in the higher members of the group, the Infusoria, there are some specializations corresponding to these functions, yet there are no distinct tissues appropriated to them. Similarly when we pass from simple types to compound types—from Protozoa to Metazoa. The animals known as Cœlenterata are characterized in common by the possession of a part which accumulates energy more or less marked off from the part which does not accumulate energy, but only expends it; and the Hydrozoa and Actinozoa, which are sub-divisions of the Cœlenterata, are contrasted in this, that in the second these parts are much more differentiated from one another, as well as more complicated. Besides a completer differentiation of the organs respectively devoted to the accumulation of energy and the expenditure of energy, animals next above the Cœlenterata possess rude appliances for the transfer of energy: the peri-visceral sac, or closed cavity between the intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. And then out of this sac originates a more efficient appliance for the transfer of energy: the more highly-organized animals, belonging to whichever sub-kingdom, all of them possess definitely-constructed channels for distributing the matters containing energy. In all of them, too, the function of expenditure is divided between a directive apparatus and an executive apparatus—a nervous system and a muscular system. But these higher sub-kingdoms are clearly separated from one another by differences in the relative positions of their component sets of organs. The habitual attitudes of annulose and molluscous creatures, is such that the neural centres are below the alimentary canal and the hæmal centres above. And while by these traits the annulose and molluscous types are separated from the vertebrate, they are separated from each other by this, that in the one the body is "composed of successive segments, usually provided with limbs," but in the other, the body is not segmented, "and no true articulated limbs are ever developed."
The sub-kingdoms being thus distinguished from one another, by the presence or absence of specialized parts devoted to fundamental functions, or else by differences in the distributions of such parts, we find, on descending to the classes, that these are distinguished from one another, either by modifications in the structures of fundamental parts, or by the presence or absence of subsidiary parts, or by both. Fishes and Amphibia are unlike higher vertebrates in possessing branchiæ, either throughout life or early in life. And every higher vertebrate, besides having lungs, is characterized by having, during development, an amnion and an allantois. Mammals, again, are marked off from Birds and Reptiles by the presence of mammæ, as well as by the form of the occipital condyles. Among Mammals, the next division is based on the presence or absence of a placenta. And divisions of the Placentalia are mainly determined by the characters of the organs of external action.
Thus, without multiplying illustrations and without descending to genera and species, we see that, speaking generally, the successively smaller groups are distinguished from one another by traits of successively less importance, physiologically considered. The attributes possessed in common by the largest assemblages of organisms, are few in number but all-essential in kind. Each secondary assemblage, included in one of the primary assemblages, is characterized by further common attributes that influence the functions less profoundly. And so on with each lower grade.
§ 103. What interpretation is to be put on these truths of classification? We find that organic forms admit of an arrangement everywhere indicating the fact, that along with certain attributes, certain other attributes, which are not directly connected with them, always exist. How are we to account for this fact? And how are we to account for the fact that the attributes possessed in common by the largest assemblages of forms, are the most vitally-important attributes?
No one can believe that combinations of this kind have arisen fortuitously. Even supposing fortuitous combinations of attributes might produce organisms that would work, we should still be without a clue to this special mode of combination. The chances would be infinity to one against organisms which possessed in common certain fundamental attributes, having also in common numerous non-essential attributes.
Nor, again, can any one allege that such combinations are necessary, in the sense that all other combinations are impracticable. There is not, in the nature of things, a reason why creatures covered with feathers should always have beaks: jaws carrying teeth would, in many cases, have served them equally well or better. The most general characteristic of an entire sub-kingdom, equal in extent to the Vertebrata, might have been the possession of nictitating membranes; while the internal organizations throughout this sub-kingdom might have been on many different plans.
If, as an alternative, this peculiar subordination of traits which organic forms display be ascribed to design, other difficulties suggest themselves. To suppose that a certain plan of organization was fixed on by a Creator for each vast and varied group, the members of which were to have many different modes of life, and that he bound himself to adhere rigidly to this plan, even in the most aberrant forms of the group where some other plan would have been more appropriate, is to ascribe a very strange motive. When we discover that the possession of seven cervical vertebræ is a general characteristic of mammals, whether the neck be immensely long as in the giraffe, or quite rudimentary as in the whale, shall we say that though, for the whale's neck, one vertebra would have been equally good, and though, for the giraffe's neck, a dozen would probably have been better than seven, yet seven was the number adhered to in both cases, because seven was fixed upon for the mammalian type? And then, when it turns out that this possession of seven cervical vertebræ is not an absolutely-universal characteristic of mammals (there is one which has eight), shall we conclude that while, in a host of cases, there was a needless adherence to a plan for the sake of consistency, there was yet, in some cases, an inconsistent abandonment of the plan? I think we may properly refuse to draw any such conclusion.
What, then, is the meaning of these peculiar relations of organic forms? The answer to this question must be postponed. Having here contemplated the problem as presented in these wide inductions which naturalists have reached; and having seen what proposed solutions of it are inadmissible; we shall see, in the next division of this work, what is the only possible solution.