We approach the subject of true adaptations, that is, of adapting processes, as soon as any kind of variation in functioning occurs which corresponds to a variation of any factor of the medium in the widest sense. But even here our work is by no means done by simply showing such a correspondence of outer and inner variations. We know very well already, from our former studies, that now we are faced by a further problem, that we are faced by the question whether we have to deal with simple primary kinds of adaptations or with the far more important secondary ones.
As the discrimination between primary and secondary regulations proves indeed to be of first-rate importance, you will allow me, I hope, to summarise our chief analytical statements regarding them in a most general form. We call primary regulatory any kind of morphogenetic or functional performance, which, by its very intimate nature, always serves to keep the whole of organisation or of functions in its normal state. We call secondary regulations all features in the whole of morphogenesis or of functioning which serve to re-establish the normal state after disturbances along lines which are outside the realm of so-called normality. This analytical discrimination will help us very much to a proper understanding of physiology. But before we turn to apply our definitions to actual facts, another preliminary problem has to be solved.
ON CERTAIN PRE-REQUISITES OF ADAPTATIONS IN GENERAL
We are thinking of the general and important question, what types of adaptations may be expected in the field of physiology and whether there may be certain classes of regulatory events which possibly might be expected to occur in the organism on a priori grounds, but which, nevertheless, are to be regarded as impossible after a more intimate analysis of its nature, even at the very beginning? Or, in other words, to what kinds of changes of the medium will an organism be found able or unable to adapt itself?
We know that the state of functioning must be altered in order to call forth any sort of adaptation at all. Now, there can be no doubt that a priori it would seem to be very useful for the organism, if it never would let enter into its blood, lymph, etc., be it through the skin or through the intestine, any chemical compound that would prove to be a poison afterwards. In fact, a man, judging on the principle of the general usefulness of all the phenomena of the living, might suppose that there would exist a sort of adaptation against all poisons to the extent that they would never be allowed to enter the real interior of the body. We know that such reasoning would be incorrect. But we also can understand, I suppose, that an a priori analysis of a more careful kind would have reasoned differently. How could the functional state of the organism be changed, and how, therefore, could adaptation be called forth by any factor of the medium which had not yet entered the organism, but was only about to enter it? Not at all therefore is such a regulation to be expected as we have sketched; if there is to be any adaptation to poisons, it only can occur after the poison has really acted in some way, and in this case we shall indeed find regulations.
You may perhaps regard this discussion as a little too academical and hair-splitting, but here again it was for the sake of ensuring a perfectly sound foundation of our chief principles that I undertook it. Very often, indeed, the question has been raised by the defenders of a mechanistic theory of life, Why then did the organisms not reject all poisons from the very beginning? We now may reply to that only—how could they do so? How could they “know” what is a poison and what is not, unless they had experienced it?—if we are allowed for a moment to use very anthropomorphistic language.
We repeat, therefore, that the functional conditions of the organism must have been actually changed in order that an adaptation may occur. Nothing is more essential to a clear understanding of our problems than to keep fully in mind the exact sense of this definition.
ON CERTAIN GROUPS OF PRIMARY PHYSIOLOGICAL ADAPTATIONS
General Remarks on Irritability.—Turning now to more special groups of problems concerning physiological adaptations, let us begin with the primary class of them, and let us first say a few words on a subject which occasionally has been regarded as the basis of physiological regulation in general. I refer to a most important fact in the general physiology of irritability. Irritability of any kind is known to be re-established, after it has been disturbed by the process of reacting to the stimulus, and in certain cases, in which two different—or rather two opposite—kinds of reactions are possible on the same substratum, which increase with regard to one process whilst decreasing at the same time with regard to the other. The irritability of the muscle or of the leaves of Mimosa is a very good instance of the first case, whilst the second more complicated one cannot be illustrated better than by what all experience has taught us about the irritability of the retina. The retina is more irritable by green rays and less by red ones the more it has been stimulated by the latter, and more sensitive to light in general the more it has been exposed to darkness; and something very similar is true, for instance, as regards phototactic irritability in plants, all these phenomena being in relation to the so-called law of Weber.[91]