Physiological selection

“ Every organ should be in due proportion, and the welfare of the creature as a whole is interfered with by excess as well as by defect. It may be directly interfered with—as for instance by too big an eyelid; and it may be indirectly interfered with, where the organ is large, by needless weight and cost of nutrition. In the second place the question which here concerns us is not what natural selection will do with variations. We are concerned with the previous question—What variations will arise? An organ varies in all ways ”

“ A place of enfeebled nutrition is commonly chosen—not unfrequently a place where a local injury has occurred. Now if we extend this principle, well recognized in pathological processes, to physiological processes, we may infer that where an adaptive modification has so reacted on the blood as to leave some matter to be got rid of, the deposit of this, initiated at some place of least resistance, may produce a local structure which eventually becomes a species-mark. ”

“ If the average amount of food which an animal can assimilate every day remains constant for a considerable time, it follows that a strong influx towards one organ must be accompanied by a drain upon others, and this tendency will increase, from generation to generation, in proportion to the development of the growing organ, which is favoured by natural selection in its increased blood-supply, etc.; while the operation of natural selection has also determined the organ which can bear a corresponding loss without detriment to the organism as a whole. ”

“ That certain organs of nutrition and respiration and locomotion are repeated in each segment of a dorsibranchiate annelid, may be regarded as functionally advantageous for a creature following its mode of life. But why should there be a hundred or even two hundred pairs of ovaries? This is an arrangement at variance with that physiological division of labour which every organism profits by—is a less advantageous arrangement than might have been adopted. That is to say, the hypothesis of a designed adaptation fails to explain the facts. ”

“ It is sufficient for our purposes to know that they may have set up certain internal stimuli which may modify the behavior of the organism in which they reside, and that the “physiological state” of the organism at the time of external or internal stimulation will condition the response. This physiological condition may be dependent on the general metabolic equilibrium of the animal, or on the extent of previous stimulation by means of the same or different agents. Thus the organism may not always react in the same way to the same stimulus. ”

“ Therefore, as nervous tissue is, physiologically speaking, the most valuable of all materials, we are forced to conclude that natural selection ought strongly to have opposed the evolution of such organs, unless from the first moment of their inception, and throughout the whole course of their development, they were of some such paramount importance as biologically to justify so unexampled an expenditure. ”

“ Pathology, therefore, is really nothing else than modified physiology. Physiological development is the evolution of organs and the growth of organisms under the impression of natural external conditions; pathological development is the evolution of organs and the growth of organisms under the impression of unnatural external conditions. Adaptation of structure to function—of organisms to surrounding media—is the designed conservative purpose of both kinds of growth. ”

“ The more healthy and perfect her organization, and the better the balance of all her organs, the sounder and the more perfect will be the development of her offspring. The health and life of the child demand it.
This theory of human increase derives strong evidence from an analogous law in the animal and vegetable kingdoms. It is well known that great improvements have been made within the present century in domestic animals, and this, too, by the application of physiological laws. ”

“ It is clear that the organism that best uses its organs has, other things equal, the best chance of survival. It will therefore hand on to its offspring germinal matter with an inherent tendency to make vigorous use of its faculties.
Those who argue thus deny that the body-cells can in any way affect the germ-cells. To account for any continuous increase in faculty, they invoke variation and the selection of favourable varieties. What, then, we may now ask, is, on their view, the mode of origin of variations? ”

“ In foregoing chapters we have constantly held the problems of evolution in view, and in special sections directed attention to them. But the subject is so central to modern thought and discussion, that some further consideration of certain aspects of the evolutionary process and products will fitly serve to bring our inquiry to a conclusion.
We must accept, as a datum from the physiological point of view, the fact that protoplasm does respond to stimuli,—that it possesses the fundamental property of irritability. It is a substance that is in a state of unstable equilibrium. ”

“ But it is not equally manifest that, on this hypothesis, alterations of structure caused by alterations of function must be transmitted to offspring. It is not obvious that change in the form of a part, caused by changed action, involves such change in the physiological units throughout the organism that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. ”

“ He who admits the principle of sexual selection will be led to the remarkable conclusion that the nervous system not only regulates most of the existing functions of the body, but has indirectly influenced the progressive development of various bodily structures and of certain mental qualities. ”

“ The complete disappearance of a rudimentary organ can only take place by the operation of natural selection; this principle will lead to its elimination, inasmuch as the disappearing structure takes the place and the nutriment of other useful and important organs. ”

“ If we seek a clue in those processes by which correlations are brought about—the physiological actions and reactions—we may at once see that any organic modification, be it adaptive or not, must entail secondary modifications throughout the rest of the organism, most of them insensible but some of them sensible. The competition for blood among organs, referred to above, necessitates that, other things remaining the same, the extra growth of any one tells on all others, in variable degrees according to conditions, and may cause appreciable diminutions of some. ”

“ If we refer adaptation to selection, we have also to trace back to this source the origin of the organic combinations which make up the various tissues of the body and which go by the collective name of muscular, nervous, glandular substance, etc. ”

“ By induction, as well as by inference from the hypothesis of natural selection, we know that there exists a balance among the powers of organs which habitually act together—such proportions among them that no one has any considerable excess of efficiency. We see, for example, that throughout the vascular system there is maintained an equilibrium of the component parts: in some cases, under continued excess of exertion, the heart gives way, and we have enlargement ”

“ As modifications of corporeal structure arise from, and are increased by, use or habit, and are diminished or lost by disuse, so I do not doubt it has been with instincts. But I believe that the effects of habit are in many cases of subordinate importance to the effects of the natural selection of what may be called spontaneous variations of instincts;—that is of variations produced by the same unknown causes which produce slight deviations of bodily structure. ”

“ They could affect only one or certain organs, because by selection only the chosen organ or organs knew the code, as it were. The chemical system is much the older system, and preceded the nerve system by aeons of time. The whole system, viscera, visceral nerves and the endocrines gradually united into a complete autonomous organism within the organism, and as such functions as the vegetative apparatus. ”

“ No individual will be able to entirely avoid this extra use, and each will endeavour to accommodate itself to the new conditions according to its power. The amount of this power depends upon the predisposition of the germ; and natural selection, while it apparently decides between individuals of various degrees of strength, is in truth operating upon the stronger and weaker germs.
But the very conclusions which have been drawn from the increase of activity which has arisen from exercise, must also be drawn from the instances of atrophy or degeneration following from the disuse of organs. ”

“ It is scarcely possible to distinguish between the supposed effects of such compensation of growth, and the effects of long-continued selection, which may at the same time lead to the augmentation of one part and the diminution of another. There can be no doubt that an organ may be greatly increased without any corresponding diminution in the adjoining parts. ”

“ And, to begin, let us ask, "How shall we know when physiological development is following its natural course, and when it is not?" There are two striking characteristics which can always here be taken as guides, viz., slowness of organic change, and latency. The physiological growth of tissue is always gradual in its progress, both as regards change of shape, bulk, and variation in physical properties. These go on with an insidious progression that is, for the most part, quite imperceptible to ordinary methods of observation. Natura non facet saltem. ”

“ That among animals physiological development is furthered by the multiplication of effects, we have lately seen proved by the many changes in other organs, which the growth or modification of each excreting and secreting organ initiates. ”

“ But the laws of natural and sexual selection are perpetually operating among those descendants. In progress of time there comes to be developed another organism, which has a wider range of nervous action; and, as this ceaseless process of modification and improvement goes on, there is developed still another nervous organization which acts with still more varied movements. As the different species of animals become evolved out of those that have gone before, the expansion of nervous organization goes on ”

“ Among these is sexual selection, and, taking a broad view, we may also include selection by man. In order, then, to the attainment of greater exactness, may we not give a name to that form of natural selection which has been potent in bringing about the variations in the characteristics of those germs to which the differences in the forms of disease are due? Such a term may be found, I would suggest, in "tissue-selection," as indicating the special means whereby the constitution of the germs has been modified. ”

“ It may be said that {333}admitting functional change to be the initiator of variation—granting that the physiological units of an organism long subject to new conditions, will tend to become modified in such way as to cause change of structure in offspring; yet there will still be no cause of the supposed heterogeneity among the physiological units of different individuals. ”

“ These advances consist in the aggregation of units which were previously independent and exposed to the full normal action of natural selection, and the two instances are, of course, the passage from the unicellular to the multicellular, and from the solitary to the social.
It is obvious that in the multicellular organism individual cells lose some of the capacities of the unicellular—reproductive capacity is regulated and limited, nutrition is no longer possible in the old simple way and response to stimuli comes only in certain channels. ”

“ Growth of the vital organs in this way may modify the distinctive form of any vital organ, brain or lungs, and thus as a consequence of modification of the internal structures due to changes of food and habit, bring a new group of animals into existence. And just as the group of animals ceases to predominate after a time, so there comes a limit to the continued internal development of vital structures as their energy fails, for each organ behaves to some extent like an independent organism. ”

“ Hence natural selection, in destroying the least fitted individuals, destroys those which from the germ were feebly disposed. Thus the result of exercise during the individual life does not acquire so much importance, for, as compared with differences in predisposition, the amount of exercise undergone by all the individuals of a species becomes relatively uniform. The increase of an organ in the course of generations does not depend upon the summation of the exercise taken during single lives, but upon the summation of more favourable predispositions in the germs. ”

“ In all organized living matter, three great fundamental actions are uninterruptedly going forward, namely, capillary circulation, nutrition, and secretion. In man, and in those animals occupying a place near him in the zoological scale, a fourth action, derived from the nervous system, is superadded, which exercises a powerful influence and control over the others. ”

“ The facts of acclimatization and immunity betray an extraordinary adaptability of the organism to a change in its environment; an organ will adapt itself structurally to an alteration, quantitative or qualitative, of function [Roux’s ‘Functional Adaptation’] ; lost parts can be regenerated; and then there is the physiology of the nervous system. ”

“ Evolution of the race proceeds by the law of natural selection. Thus, if those who are born with great vigor and strong lungs are enabled to live where their weak-lunged neighbors will die, the result will be that their offspring, having greater lung capacity, will form a race with increased lung capacity. ”