Plasticity (biology)

“ 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. ”

“ Natural selection, then, will ensure the damping down of the swing of all the pendulums within comparatively narrow limits.
But now suppose that the conditions of the environment somewhat rapidly change. Congenital variations will not be equal to the occasion. The swing of the pendulums concerned cannot be rapidly augmented. Here individual plasticity steps in to save some of the members of the race from extinction. They adapt themselves to the changed conditions through a modification of the bodily tissues. ”

“ An illustration of this is the contrast between anabolism and katabolism, as is also the increased plasticity manifested in the prolongation of childhood. Plastic brain cells do not result in a single mental trait, but in a change in the whole range of mental activity. The races of slow maturity differ from those rapidly maturing in many traits, and yet they may all be the result of a single variation involving the increased plasticity of brain cells. ”

“ 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. ”

“ Inanimate objects such as iron, rocks, sulphur, oxygen, etc., have no plasticity. Plants have very little possibility of forming habits. Lower animals have somewhat more, and higher animals still more, while man has the greatest possibility of forming habits. ”

“ 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. ”

“ The muscles may acquire unusual strength by use and exercise; the nerve-centres may learn certain tricks of behaviour in the course of individual life; and other structures may be similarly accommodated to the conditions which affect them. To such modifications of structure or function in the organs or parts the term acquired is primarily applied. The tissues have thus a certain amount of organic plasticity, through which they are adjusted to a range of circumstances varying in extent. They are able to acquire new modes of behaviour. ”

“ If we consider the number of obstacles that have to be overcome in existence within other animals, and how difficult and how much a matter of chance it must be even to reach to such a place as, for instance, the intestine, the liver, the lungs, or even the brain or the blood of another animal, and when, on the other hand, we know how exactly things are now regulated for every parasitic species so that its existence is secured notwithstanding its dependence upon chance, we must undoubtedly form a high estimate of the plasticity of the forms of life and their adaptability. ”

“ Whatever the extent of these bodily changes may be, if we grant the correctness of our inferences in regard to the plasticity of human types, we are necessarily led to grant also a great plasticity of the mental make-up of 65human types. We have observed that features of the body which have almost obtained their final form at the time of birth show modifications of great importance in new surroundings. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 us reasons for associating an inferior and more rapidly-completed mental type with a relatively automatic nature. Lowly-organized creatures, guided almost entirely by reflex actions, are in but small degrees changeable by individual experiences. As the nervous structure complicates, its actions become less rigorously confined within preëstablished limits ”

“ 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 have lost that plasticity, that freedom of growth, which characterizes the simpler and more primitive forms of life; in their case the power of producing fresh organisms entire from a single fragment, once diffused equally over the whole body, is now confined to certain specialized cells which, in their developed form, we know as seeds or eggs. Yet, even among animals, at a low stage of development, this original power of reproducing the whole from a single part remains inherent in the organism ”

“ The plasticity of the living material of our nervous system is, to put it briefly, the reason why we do a thing with difficulty the first time, but soon more and more easily, and finally, with sufficient practice, we do it half mechanically, or almost without any consciousness. ”

“ 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. ”

“ Experimental evidence is even proving that the general plasticity of the nervous system, which has always been considered to be transitory, is of very, very much longer duration than has been supposed. ”

“ They are plastic because they can to some extent change their habits, and changed habit, if persisted in, involves corresponding change, however slight, in the organs employed; but their plasticity depends in great measure upon their failure to perceive that they are moulding themselves. If a change is so great that they are seriously incommoded by its novelty, they are not likely to acquiesce in it kindly enough to grow to it, but they will make no difficulty about the miracle involved in accommodating themselves to a difference of only two or three per cent. ”

“ 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. ”

“ Let us suppose, however, that a group of organisms belonging to a plastic species is placed under new conditions of environment. 9. Those whose innate somatic plasticity is equal to the occasion survive. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ We have given protoplasm, a strange substance of marvellous capacities, which we call functions, and possessing a power of developing into beings of ever higher grades of organization. Environment proves to be a combination of forces working for the higher development of functions in a certain orderly sequence. And every lower function in the ascending line demands the development of the next higher. Digestion demands muscle, and muscle nerve, and nerve brain. We shall soon see that mammalian structure had to culminate in the family, and the family demands unselfishness and obedience. ”

“ In land-animals the bones of the limbs have more play in their sockets than in air or water animals, because flexibility is necessary to speed over the ground or trees, but it is an objection where the limbs are to be used as paddles against the air or water. In fact, the more we know of the skeleton and the environment of the animal, the more we see that its variations throughout the animal kingdom are the combined results of the movements of the organism itself and the opposition of the medium in which it moves. ”

“ 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. ”