Phenotypic plasticity

“ Plasticity of behaviour in special accommodation to special circumstances is of survival value; it depends upon acquired characters; it is correlated with increase in size and complexity of the cerebrum; under natural selection therefore the larger and more complex cerebrum as the organ of plastic behaviour has been the outcome of natural selection. We have thus the biological foundations for a further development of genetic psychology.
There are diversities of opinion, as Darwin showed, with regard to the range of instinct in man and the higher animals as contrasted with lower types. ”

“ The old ideas about race as something hard and fast for all time are distinctly on the decline. Plasticity, or, in other words, the power of adaptation to environment, has to be admitted to a greater share in the moulding of mind, and even of body, than ever before. But how plasticity is related to race we do not yet know. It may be that use-inheritance somehow incorporates its effects in the offspring of the plastic parents. Or it may be simply that plasticity increases with inter-breeding on a wider basis. ”

“ The study of evolution may give us a further help. We have seen how the final steps of the highest forms of animals have been in the direction of plasticity of organization: we see it in the rise of man from mammals, in higher as against more primitive levels of human culture, in great men as against ordinary men. There can be no doubt that its acquisition constitutes a step in evolutionary progress. Plasticity is needed in any new religion. And plasticity means tolerance, means the reduction of fixity of ritual, of convention, of dogma, of clericalism. ”

“ When the structure has yielded, the same inertia becomes a condition of its comparative permanence in the new form, and of the new habits the body then manifests. Plasticity, then, in the wide sense of the word, means the possession of a structure weak enough to yield to an influence, but strong enough not to yield all at once. ”

“ Rapid morphological change on the part of plastic species. When catastrophic change burst in upon the ages of uniformity, and sounded in the ear of every living thing the words 'Change, or die,' plasticity became the sole principle of salvation. ”

“ At the same time, other European nationalities exhibit changes of other kinds, all these changes, however, being in the direction of a convergence towards one and the same American type. How are we to explain these facts, supposing them to be corroborated by more extensive studies? It would seem that we must at any rate allow for a considerable plasticity in the head-form, whereby it is capable of undergoing decisive alteration under the influences of environment; not, of course, at any moment during life, but during those early days when the growth of the head is especially rapid. ”

“ At first sight an apparent contradiction arises from the admission of both heredity and adaptation; and it is undoubtedly true that a strong disposition to heredity precludes great capability of adaptation. But imagine the organism to be a plastic mass which retains the form transmitted to it by former influences until new influences modify it; the one property of plasticity will then represent capability of adaptation as well as power of heredity. ”

“ Starting as a one-celled creature, he recapitulates in the nine months of embryonic life the [Pg 45] processes of evolution that required millions of years, from the amœba to the higher vertebrates, in the evolution of the species.
At birth the baby is less developed and more plastic than the young of any other creature at its birth. This helplessness and plasticity are due to the incompleteness in development of the nervous system. It is because of this incompleteness that the physical, mental, and spiritual life can be shaped in great measure by environment. ”

“ If habits are due to the plasticity of materials to outward agents, we can immediately see to what outward influences, if to any, the brain-matter is plastic. Not to mechanical pressures, not to thermal changes, not to any of the forces to which all the other organs of our body are exposed; for nature has carefully shut up our brain and spinal cord in bony boxes, where no influences of this sort can get at them. ”

“ It is impossible not to be struck with the high degree of variability or plasticity of many of the bones. We see how erroneous the often-repeated statement is, that only the crests of the bones which give attachment to muscles vary in shape, and that only parts of slight importance become modified under domestication. No one will say, for instance, that the occipital foramen, or the atlas, or the third cervical vertebra is a part of slight importance. ”

“ Relative plasticity.—Is there any relation between the degree of mental modifiability which remains in adult life, and the character of the mental evolution in respect of mass, complexity, and rapidity? The animal kingdom at large yields reasons for associating an inferior and more rapidly-completed mental structure, with a relatively automatic [356] nature. Lowly organized creatures, guided almost entirely by reflex actions, are in but small degrees changeable by individual experiences. ”

“ Also in "l'integration croissante de la force evolutive a mesure qu'elle se partage dans les formes produites, et la decroissance proportionelle de la plasticite de ces formes a mesure qu'elles s'eloignent davantage de leur origine, et qu'elles sont mieux arretees." As they get older, they gain in fixity through the operation of the fundamental law of inheritance; but the species, like the individual, loses plasticity and vital force. ”

“ Difficult as it may be to earmark the items of the organic bequest—to say that, of the sum of energy expended in any given case of organic behaviour, so much is due to a specific draft definitely assigned in heredity for this particular purpose, and so much is contributed from the general legacy of innate plasticity,—it none the less conduces to clear thinking to emphasize the logical distinction between them, so long as it is steadily borne in mind that logical distinction does not imply biological separation. ”

“ The characters which made one species adapted for the plain, another for the mountain forest were already there; they can only be referred to personal selection, which brought about the adaptation of their ancestors in the course of ages to the conditions of their life. Something similar is true of the adaptations of the tissues; the differentiation of the individual kinds of cells is an ancient inheritance, and depends upon personal selection, but their distribution and arrangement into specially adapted tissues, so far as there is any plasticity at all, depends upon histonal selection. ”

“ But though under constant and uniform changes in the environment the net result would be only a guided variation of the original instinct, under more variable and indefinitely changing circumstances the result would be different. The higher animals exhibit an intelligent plasticity which enables them to meet the requirements of the more complex environment into which their wider life has risen; for evolution lifts the animal from narrower into progressively wider spheres of activity and behaviour, so that its environment becomes relatively more complex. ”

“ These creatures you have seen are animals carven and wrought into new shapes. To that, to the study of the plasticity of living forms, my life has been devoted. I have studied for years, gaining in knowledge as I go. I see you look horrified, and yet I am telling you nothing new. It all lay in the surface of practical anatomy years ago, but no one had the temerity to touch it. ”

“ In a word, the living cell {303} presents a marvellous plasticity, which exerts itself without the slightest disturbance by minimal deviations of forces due to dissymmetrical influence. Ah, if spontaneous generation were only possible! If we could only create living matter, raise up in the midst of inactive mineral material a living cell, then it would be easy for us to understand something more of vital manifestations and to comprehend better the mystery of dissymmetry. ”

“ Plasticity, then, is that quality which, in suddenly enforced physical change, is the key to survival and prosperity. And the survival of the plastic, that is, of the rapidly and healthily modifiable during periods when terrestrial revolution offers to species the rigorous dilemma of prodigious change or certain death, is a widely different principle from the survival of the fittest in a general biological battle during terrestrial uniformity. ”

“ While throughout the immense majority of the semi-solid bodies, namely, the organic bodies, the proclivity to a re-arrangement of parts is so comparatively great, as to be usually taken for a distinctive characteristic of them. Among organic bodies themselves, we may trace contrasts having a like significance. It is an accepted generalization that, other things equal, the rate of Evolution is greatest where the plasticity is most 338marked. ”

“ They were his environment, these men, and they were moulding the clay of him into a more ferocious thing than had been intended by Nature. Nevertheless, Nature had given him plasticity. Where many another animal would have died or had its spirit broken, he adjusted himself and lived, and at no expense of the spirit. ”

“ Hence, if the original and primitive vertebrate animal or root-stock were like the larval sea-squirt, as we suppose it to have been, its body would be transparent, and its eye or eyes, situated on its brain, would receive light-rays through its clear body. But, as the evolution of the vertebrate race proceeded, the tissues became firmer and denser. By "natural selection"—or, in other words, by the exercise of accommodating power to function the eyed region of the brain would tend to grow more and more toward the body's surface, to receive the rays of light. ”

“ We should bear in mind that selection has not been applied to fix any character in the skeleton, and that the animals have not had to support themselves under uniform habits of life. We cannot account for most of the differences in the skeleton; but we shall see that the increased size of the body, due to careful nurture and continued selection, has affected the head in a particular manner. Even the elongation and lopping of the ears have influenced in a small degree the form of the whole skull. ”

“ Many of these connections are inborn and seem to be as much a part of the heredity of the individual and the race as stature, the color of eyes and hair, or any other physical characteristic. Hence the nervous system is characterized by a multitude of more or less perfectly developed preorganized reactions which are a part of the individual's heredity. These preorganized reactions have been established through the operation of selection, biologists tell us, upon variations in the hereditary elements, in the same way in which the bodily characteristics of the species have been established. ”

“ If the prerogatives of Selection within the germ, of segrega­tion of unit-characters and dominance, and of mutations are not unlimited in the construc­tion of organisms, there still remains a sphere of action for the initiating power of the nervous system. Bones grow and change their form in response to increased or altered muscular action on them, and it is necessary to look back a stage further in the story to the neural changes however produced. ”

“ Thus the multitude of small individual variations, which probably occur in every species, but which strike us most in Man—the differences in the development of mouth, nose, and eyes, in the hair, in the colour of skin, &c., as far as they are without significance in the struggle for existence—depend upon processes of germinal selection, which permitted the greater development of one group of determinants, or of one kind of biophor in one case, of another in another. ”

“ 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. ”

“ Art also steps in to help the process of the higher Evolution. Our physicians have discovered that the small and tender sides of an infant Polygon of the higher class can be fractured, and his whole frame re-set, with such exactness that a Polygon of two or three hundred sides sometimes—by no means always, for the process is attended with serious risk—but sometimes overleaps two or three hundred generations, and as it were doubles at a stroke, the number of his progenitors and the nobility of his descent. ”

“ In back-boned animals from frog to man there is a great variety of fore-limb, adapted for running, swimming, flying, grasping, and so forth, but throughout there is a unity of structure and development. There are the same fundamental bones and muscles, nerves and blood vessels, and the early stages are closely similar. So it is throughout organic nature; there is unity of type, maintained under extreme dissimilarities of form and mode of life. ”

“ Thus through the innumerable symbols supplied by environment the distance ceptors drive this or that animal according to the type of brain pattern and the particular state of threshold which has been developed in that animal by its phylogenetic and ontogenetic experiences. The brain pattern depends upon his phylogeny, the state of threshold upon his ontogeny. Each BRAIN PATTERN is created by some particular element in the environment to which an adaptation has been made for the good of the species. ”