The Evolution Theory, Vol. 2 of 2

by August Weismann

But in many cases this is not enough, namely, in all cases in which the degree of intelligence is not sufficiently high, or where the flight movement must follow so rapidly that it would be too late if it had to be regulated by the will, as, for instance, the shutting of the lids when the eye is threatened, or the flight of the fly or the butterfly when an enemy approaches. Both fly and butterfly would be lost in every case if they had voluntarily to set the flight-movement going after they became conscious of danger. If they had first of all to find out from whom danger threatened no individual would escape an early death, and the species would die out. But they possess an instinct which impels them to fly up with lightning speed, and in an opposite direction, whenever they have a visual perception of the rapid approach of any object of whatever nature. For this reason they are difficult to catch. I once watched the play of a cat, ordinarily very clever at catching, as she attempted to seize a peacock-butterfly (Vanessa Io), which settled several times on the ground in front of her. Quietly and slowly she crept within springing distance, but even during the spring the butterfly flew up just before her nose and escaped every time, and the cat gave it up after three attempts.

In this case the beginning of the action cannot lie in a voluntary action, for the insect cannot know what it means to be caught and killed, and the same is true of innumerable still lower animal forms, the hermit-crabs and the Serpulids, which withdraw with lightning speed into their houses, and so forth. It seems to me important theoretically, that the same action can be liberated at one time by the will, at another by the inborn instinct-mechanism. In both cases quite similar association-changes in the nerve-centres must lie at the root of the animal's action, but in the first case these are developed only in the course of the individual life by exercise, while in the second they are inborn. In the former, they are confined to the individual, and must be acquired in each generation by imitation of older individuals (tradition) and by inference from experience, in the latter they are inherited as a stable character of the species.

It has been maintained by many that the origin of instincts through processes of selection is not conceivable, because it is improbable that the appropriate variations in the nervous system, which are necessary for the selective establishment of the relevant brain-mechanism, should occur fortuitously. But this is an objection directed against the principle of selection itself, and one which points, I think, to an incompleteness in it, as it was understood by Darwin and Wallace. The same objection can be made to every adaptation of an organ through natural selection; it is always doubtful whether the useful variations will present themselves, as long as they are due solely to chance, as the discoverers of the selective principle assumed. We shall attempt later to fill up this gap in the theory, but, in the meantime, I should like to point out that the process of selection offers the only possible explanation of the origin of instincts, since their origin through modifications of voluntary actions into instinctive actions, with subsequent transmission of the instinct-mechanism due to exercise in the individual life, has been shown to be untenable.

If any one is still unconvinced of this, I can only refer to the cases we have already discussed of instincts which are only exercised once in a lifetime, since, in these, the only factor that can transform a voluntary action into an instinctive one is absent, namely, the frequent repetition of the action. In this case, if any explanation is to be attempted at all, it can only be through natural selection, and as we have assumed once for all that our world does admit of explanation, we may say, these instincts have arisen through natural selection.

Even though it may be difficult to think out in detail the process of the gradual origin of such an instinctive activity, exercised once in a lifetime, such as that, for instance, which impels the caterpillar to spin its intricate cocoon, which it makes only once, without ever having seen one, and thus without being able to imitate the actions which produce it, we must not push aside the only conceivable solution of the problem on that account, for then we should have to renounce all hope of a scientific interpretation of the phenomenon. We may ask, however, whether there is not something lacking in our present conception of natural selection, and how it comes about that useful variations always crop up and are able to increase.

But if we must explain, through natural selection, such complex series of actions as are necessary to the making of the cocoon of the silkworm or of the Saturnia moth (Saturnia carpini), what reason have we for not referring other instincts also to selection, even if they be repeated several times, or often, in the course of a lifetime? It is illogical to drag in any other factor, if this one, which has been proved to operate, is sufficient for an explanation.

Thus, as far as instincts are concerned, there is no necessity to make the assumption of an inheritance of functional changes, any more than there is in regard to any purely morphological modifications. As the instincts only exercised once show us that even very complicated impulses may arise without any inheritance of habit, that is, without inheritance of functional modification, so there are among purely morphological characters not a few which, though effective, are purely passive, which are of use to the organism only through their existence, and not through any real activity, so that they cannot be referred to exercise, and therefore cannot be due to the transmission of the results of exercise. And, if this be the case, then transformations of the most diverse parts may take place without the inheritance of acquired characters, that is, of functional modifications, and there is no reason for dragging in an unproved mode of inheritance to explain a process which can quite well be explained without it. For if any part whatever can be transformed solely through natural selection, why, since there is general variability of all parts, should this be confined to the passive organs alone, when the active ones are equally variable, and equally important in the struggle for existence?

There are, indeed, many of these passive parts among animals; I need only recall the coloration of animals, the whole set of skeletal parts, so diversely formed, of the Arthropods, the legs, wings, antennæ, spines, hairs, claws, and so on, none of which can be changed by the inherited results of exercise, because they are no longer capable of modification by exercise; they are ready before they are used; they come into use only after they have been hardened by exposure to the air, and are no longer plastic; they are at most capable of being used up or mutilated. Finally, even so convinced an advocate of the Lamarckian principle as Herbert Spencer has stated that among plants the great majority of characters and distinctive features cannot be explained by it, but only through the principle of selection; all the diverse protective arrangements of individual parts, like thorns, bristles, hairs, the felt-hairs of certain leaves, the shells of nuts, the fat and oil in seeds, the varied arrangements for the dispersal of seeds, and so on, all operate by their presence alone, not through any real activity which causes them to vary, and the results of which might be transmitted. An acacia covered all over with thorns seldom requires to use its weapons even once, and if a hungry ruminant does prick itself on the thorns it is only a few of these which are thus 'exercised,' the rest remain untouched.

But since all these parts have originated notwithstanding their passivity, there must be a principle which evokes them in relation to the necessities of the conditions of life, and this can only be natural selection, that is, the self-regulation of variations in reference to utility. And if there is this principle, we require no other to explain what is already explained.