The stress of the struggle for existence falls upon the body; and instinctive or intelligent behaviour is a means to its preservation in the struggle for existence. According as it survives or not, will the samples of germinal substance it contains fulfil their biological end or perish with it. Natural selection secures the survival of those animals which bear the seed from which their like will be developed.
On this view all variation arises within the germinal substance, but it is manifested in the body which is its product. How variations arise we do not know with any exactness of detail. That the germinal substance is influenced in its nutrition and in other ways by the surrounding tissues is highly probable; and this influence may lead to changes which are the source of variations; but it is very doubtful whether such influence can be what we before termed “homœopathic.”[202] It is improbable that the formation of the nerve-connections involved in intelligent behaviour which has grown habitual through repetition, can so influence the germinal cells as to give rise to variations of like nature. In other words, acquired habit is probably not a direct determinant of an inherited variation of like nature in instinctive behaviour. Apart from such influence the only source of variations which can be assigned is either the differential division of nuclei in preparation for the process of fertilization,[203] or the process of fertilization itself. The union of perhaps differentiated germinal substance from two distinct parents affords the opportunities for the admixture and compounding of hereditary qualities in the two samples, from which variations favourable or the reverse may arise.
It is now generally recognized, however, that the origin of variations is a problem quite distinct from that of the survival of those whose direction is favourable to that end. The theory of natural selection, as such, does not pretend to offer any explanation of the manner in which variations arise; though of course a complete theory of organic evolution must assign the antecedents and conditions of organic progress in all its varied phases. We know that variations do occur; we know, too, that more individuals are born than survive to procreate their kind; and, on the theory of natural selection, we draw from these data the conclusion that, on the average, the animals that escape elimination are those in which the variations are of such a nature as to conduce to this end.
It will be seen that, on the hypothesis of organic heredity, thus briefly sketched, continuity can, in strictness and, as we may phrase it, in its first intent, only be predicated of the germinal substance; but that this substance gives rise to products—active vigorous animals behaving in certain ways, each after his kind—which hold similar germinal substance in trust for future use. Natural selection deals with the trustees; and if they succumb, that which they hold in trust is lost. To put the matter in another way: Nature says to the germinal substance, “By your products you must be judged in accordance with the criterion of utility and efficiency.” Practical use in the give and take of active life is the touchstone of all behaviour which makes for survival. This being secured, there may be a balance of behaviour for other purposes. But in animals the balance is not of large amount, and other purposes have not taken form and direction. It should be clearly noticed that, on the hypothesis we are considering, use is the test of survival, and though it is not the direct cause of variations, it affords their sanction in survival. That animal escapes elimination whose behaviour is of practical use; and it holds in trust for the future a store of germinal substance from which is produced a successor capable of behaving in like manner.
The whole drama of organic evolution may be regarded as the realization in a succession of individuals of the evolving potentiality of continuous lines of germinal substance. The successive individuals die—but the germinal substance lives on in their heirs, if they have any. In virtue of what intimate and hidden structure or disposition of parts the germ possesses this potentiality we do not know. The ovum of a dog is a microscopic speck less than one-hundredth of an inch in diameter; the sperm is far more minute. They unite, and their nuclei coalesce. The cellular product divides and subdivides. The cell colony absorbs nutriment from the maternal tissues. Division proceeds apace, and the cells are marshalled and ordered in embryological development; definite tissues are formed; the stages of their genesis can be predicted with accuracy; and in due time a puppy is born which shall grow to the likeness of its parents and behave as they behaved. We can trace the succession of events; we see that they form a related series; we have good reason for believing that the state of matters at any one moment is the antecedent condition of the state of matters at the succeeding moment. More than this science cannot say. The underlying cause is, for science, hidden in the mists of the unknown. Even for metaphysics it is but part of the force that beats through the universe and makes it not a chaos but a cosmos—a force known to us only in its effects.
It will thus be seen that the conception of continuity in organic evolution has, broadly considered, a threefold aspect. First, there is the continuity of the germinal substance through whose reproductive behaviour under the appropriate conditions embryological development occurs; secondly, there is continuity in this embryological development, stage by stage, from the fertilized ovum to the adult which is its final product and expression; thirdly, there is continuity in these final products, in the animals whose organic, instinctive, and intelligent behaviour lie open to our study and investigation. The first is germinal, the second developmental, the third evolutional continuity.
Before attempting to summarize some of the contributions afforded by our inquiry towards the doctrine of continuity in the last of these three aspects, we must pause for a moment to consider how far and in what sense continuity can be predicated of mental development.
We have regarded the conscious situation as the psychical aspect of a nerve-situation in the sensorium; and the nervous system, capable of behaving in this way, is in developmental continuity with the germinal substance of the fertilized ovum. But what shall we say with regard to the psychical aspect? Two hypotheses seem open to us, each of which presents difficulties, but of different kinds. The first is, that when the organic development of the nervous system reaches a certain level and order of complexity consciousness emerges, how and whence we know not. The second is, that consciousness is developed from sentience, which is the concomitant of all organic behaviour; which accompanies life wherever it occurs and therefore shares the continuity of the germinal substance.
The difficulty inseparable from the first hypothesis, is that it is contrary to the analogy of all that we know or infer elsewhere throughout the realm of nature. Huxley[204] likened its emergence to the production of heat when an iron bar is struck by repeated blows of the hammer. But this analogy will not hold; for heat is a mode of energy, and only emerges through the transformation of other and pre-existing modes of energy. A certain amount of the energy of motion in the massive hammer-head is transferred to the iron rod, and assumes the form of that molecular vibration which we call heat. And by what amount the one is the gainer, by that amount is the other the loser. But we have no reason to suppose that the like takes place in the origin of the mental concomitants of neural changes. No portion of the brain’s store of physical energy is drained off to form the rivulet of consciousness. Now, whenever we speak of a product elsewhere in nature, we mean a specialized bit of something pre-existent. Water is the product of pre-existing oxygen and hydrogen. Heat is the product of other forms of energy. But this is not so on the first hypothesis, according to which consciousness emerges when the functional activity of the nervous system reaches a certain level and order of complexity. The mental concomitants are not “products,” in the recognized sense of the term. Furthermore, although on this hypothesis we may still speak of what was termed above evolutional continuity in the mental concomitants, there is nothing analogous to either developmental or germinal continuity.
On the second hypothesis, according to which sentience is the concomitant of all organic behaviour, such developmental and germinal continuity, or their analogues in the psychical order of being, are rendered conceivable. Consciousness is regarded as a developed form of sentience. But the sentience is wholly hypothetical. It is at best a “may be,” and its existence is incapable of proof. And science is rightly impatient of hypotheses the validity of which cannot in any way be verified. Our safest course, therefore, is to accept that which is common to both hypotheses, evolutional continuity, and for the rest to be content with a confession of ignorance.