Perhaps, however, M. Naudin does not mean that "evolutive force," or the force of vitality, is really homologous with common physical force, but only something which may be likened to it. In that case the parallel has only a metaphorical value, and the reason why variation must cease and species die out is still to seek. In short, if that which continues the series of individuals in propagation, whether like or unlike the parents, be a force in the physical sense of the term, then there is abundant provision in Nature for its indefinite replenishment. If, rather, it be a part or phase of that something which directs and determines the expenditure of force, then it is not subject to the laws of the latter, and there is no ground for inferring its exhaustibility. The limited vitality is an unproved and unprovable conjecture. The evolutive force, dying out in the using, is either the same conjecture repeated, or a misapplied analogy.
After all—apart from speculative analogies—the only evidences we possess which indicate a tendency in species to die out, are those to which Mr. Darwin has called attention. These are, first, the observed deterioration which results, at least in animals, from continued breeding in and in, which may possibly be resolvable into cumulative heritable disease; and, secondly, as already stated (p. 285), what may be termed the sedulous and elaborate pains everywhere taken in Nature to prevent close breeding—arrangements which are particularly prominent in plants, the greater number of which bear hermaphrodite blossoms. The importance of this may be inferred from the universality, variety, and practical perfection of the arrangements which secure the end; and the inference may fairly be drawn that this is the physiological import of sexes. It follows from this that there is a tendency, seemingly inherent, in species as in individuals, to die out; but that this tendency is counteracted or checked by sexual wider breeding, which is, on the whole, amply secured in Nature, and which in some way or other reenforces vitality to such an extent as to warrant Darwin's inference that "some unknown great good is derived from the union of individuals which have been kept distinct for many generations." Whether this reenforcement is a complete preventive of decrepitude in species, or only a palliative, is more than we can determine. If the latter, then existing species and their derivatives must perish in time, and the earth may be growing poorer in species, as M. Naudin supposes, through mere senility. If the former, then the earth, if not even growing richer, may be expected to hold its own, and extant species or their derivatives should last as long as the physical world lasts and affords favorable conditions. General analogies seem to favor the former view. Such facts as we possess, and the Darwinian hypothesis, favor the latter.
XIII
EVOLUTIONARY TELEOLOGY
When Cuvier spoke of the "combination of organs in such order that they may be in consistence with the part which the animal has to play in Nature," his opponent, Geoffroy St.-Hilaire, rejoined, "I know nothing of animals which have to play a part in Nature." The discussion was a notable one in its day. From that time to this, the reaction of morphology against "final causes" has not rarely gone to the extent of denying the need and the propriety of assuming ends in the study of animal and vegetable organizations. Especially in our day, when it became apparent that the actual use of an organ might not be the fundamental reason of its existence— that one and the same organ, morphologically considered, was modified in different cases to the most diverse uses, while intrinsically different organs subserved identical functions, and consequently that use was a fallacious and homology the surer guide to correct classification—it was not surprising that teleological ideas nearly disappeared from natural history. Probably it is still generally thought that the school of Cuvier and that of St.-Hilaire have neither common ground nor capability of reconcilement.
In a review of Darwin's volume on the "Fertilization of Orchids" * (too technical and too detailed for reproduction here), and later in a brief sketch of the character of his scientific work (art. IX, p. 234), we expressed our sense of the great gain to science from his having brought back teleology to natural history. In Darwinism, usefulness and purpose come to the front again as working principles of the first order; upon them, indeed, the whole system rests.
To most, this restoration of teleology has come from an unexpected quarter, and in an unwonted guise; so that the first look of it is by no means reassuring to the minds of those who cherish theistic views of Nature. Adaptations irresistibly suggesting purpose had their supreme application in natural theology. Being manifold, particular, and exquisite, and evidently inwrought into the whole system of the organic world, they were held to furnish irrefragable as well as independent proof of a personal designer, a divine originator of Nature. By a confusion of thought, now obvious, but at the time not unnatural, they were also regarded as proof of a direct execution of the contriver's purpose in the creation of each organ and organism, as it were, in the manner man contrives and puts together a machine—an idea which has been set up as the orthodox doctrine, but which to St. Augustine and other learned Christian fathers would have savored of heterodoxy.
In the doctrine of the origination of species through natural selection, these adaptations appear as the outcome rather than as the motive, as final results rather than final causes. Adaptation to use, although the very essence of Darwinism, is not a fixed and inflexible adaptation, realized once for all at the outset; it includes a long progression and succession of modifications, adjusting themselves to changing circumstances, under which they may be more and more diversified, specialized, and in a just sense perfected. Now, the question is, Does this involve the destruction or only the reconstruction of our consecrated ideas of teleology? Is it compatible with our seemingly inbore conception of Nature as an ordered system? Furthermore, and above all, can the Darwinian theory itself dispense with the idea of purpose, in the ordinary sense of the word, as tantamount to design?
From two opposing sides we hear the first two questions answered in the negative. And an affirmative response to the third is directly implied in the following citation:
"The word purpose has been used in a sense to which it is, perhaps, worth while to call attention. Adaptation of means to an end may be provided in two ways that we at present know of: by processes of natural selection, and by the agency of an intelligence in which an image or idea of the end preceded the use of the means. In both cases the existence of the adaptation is accounted for by the necessity or utility of the end. It seems to me convenient to use the word purpose as meaning generally the end to which certain means are adapted, both in these two cases and in any other that may hereafter become known, provided only that the adaptation is accounted for by the necessity or utility of the end. And there seems no objection to the use of the phrase 'final cause' in this wider sense, if it is to be kept at all. The word 'design' might then be kept for the special case of adaptation by an intelligence. And we may then say that, since the process of natural selection has been understood, purpose has ceased to suggest design to instructed people, except in cases where the agency of man is independently probable."—P.C.W., in the Contemporary Review for September, 1875, p. 657.