We meet here with one of the most characteristic and unique features of living things as contrasted with non-living things. We shall have to dismiss at once the idea that we can explain this attribute of organisms by either the selection or the mutation theory; for we find animals possessing this power that could never be supposed to have acquired it by any experience to which they have been subjected; and since it appears to be so universally present, we cannot account for it as a chance mutation that may have appeared in each species. No doubt Wolff had responses of this kind in view when he made the rather sweeping statement that purposeful adaptation is the most characteristic feature of living things. The statement appears to contain a large amount of truth, if confined to the present group of phenomena.
This power of self-regulation may confer a great benefit on its possessor. The increase in the size and strength of the muscles through use may give the animal just those qualities that make its existence easier. The increase in the power of vision, or at least of visual discrimination through use, of the power of smell and of taste, of hearing and of touch, are familiar examples of this phenomenon.
However much we may be tempted to speculate as to how this property of the animal may have been acquired, we lack the evidence which would justify us in formulating even a working hypothesis. It may be that when we come to know more of what the process of contraction of the muscle involves, the possibility of its development as a consequence of its use may be found to be a very simple phenomenon that requires no special explanation at all to account for its existence in the individual, further than that the muscles are of such a kind that this is a necessary physical result of their action. But until we know more of the physiology involved in the process, it is idle to speculate about the origin of the phenomenon.
Reactions of the Organism to Poisons, etc.
In this case also we meet with a number of responses for whose origin we can give not the shadow of an explanation. On the other hand, the cases are significant in so far as a number of them show quite clearly that the response cannot have been acquired through the experience of the organism, or the selection of those individuals that have best resisted the particular poison. This is true, because in a number of cases the poison is a substance that the animal cannot possibly have met with during the ordinary course of its life, or of that of its ancestors. It may be argued, it is true, that in the case of the poisons produced by certain bacteria the power of resistance has been acquired through the survival of the less susceptible, or more resistant, individuals. Improbable as this may be in some cases, it does not, even if it were true, alter the real issue, for it can be shown, as has just been said, that the same power of responding adaptively is sometimes shown in cases of poisons that are new to the animal.
There is no question that different individuals respond in very different degrees to these poisonous substances, and it is easy to imagine in the case of contagious diseases that a sort of selective process might go on that would bring the race up to the highest point to which fluctuating variations could be carried, even to complete immunity; but even if this were the case, it seems to be true that the moment the selection stopped the race would sink back to the former condition.
All this touches only indirectly the main point that we have under consideration, namely, the existence of this power of resistance in cases where it cannot have been the result of any educative process. Since the responses to new poisons do not appear to be in principle different from the responses to those to which the organism may have possibly been subjected at times in the past, we shall probably not go far wrong if we treat all cases on the same general footing. Whether the power of adaptation to certain substances, such as nicotine, morphine, cocaine, arsenic, alcohol, etc., is brought about by the formation of a counter-substance is as yet unproven. And while it seems not improbable that in some of these instances it may turn out that this is the case, especially for poisons of plant origin, it is better to suspend judgment on this point until each case has been established.
In recent years it has been shown that the animal body has the power of making counter-substances when a very large number of different kinds of things are introduced into the blood. We seem to be here on the threshold of a field for discovery which may, if opened up, give us an insight into some of the most remarkable phenomena of adaptation shown by living things.
It has already been pointed out that it appears to be almost a reductio ad absurdum to speak of animals adapting themselves to poisonous substances. It is curious, too, that in man at least the use of these substances may arouse a craving for the poison, or at any rate the individual may become so dependent on the poison that the depression following its disuse may lead to a desire for a repetition of the dose. The two questions that are raised here must be kept apart, for the adaptation of the individual to the poison and the so-called craving for it may depend on quite different factors. Nevertheless, it seems to be true in the case of morphine and of arsenic, and probably for some other substances as well, that if their use is suddenly stopped the individual may die in consequence. In this respect the organism behaves exactly as it does to an environment to which it has become adapted.