C. Lloyd Morgan

“ 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. ”

“ By modifications I mean those changes which are in some way wrought in the body-structure, and by variations those differences which are of germinal origin. That variation of germinal origin is a fact in organic nature is admitted on all hands, and that some variations are adaptive is also unquestioned. Transmissionists contend that modification in a particular direction in one generation is, through the transmission of the change in some way from the bodily tissues to the germinal cells, a source of variation in the same direction in the next generation. ”

“ Whereas all the other pendulums are still damped down by natural selection as before, the oscillation of the pendulum, which represents a variation in this bony structure, is no longer checked. It is free to swing as much as it can. Congenital variations in the direction of adaptive modification will be so much to the good of the individual concerned. They will constitute a congenital predisposition to that strengthening of the part which is essential for survival. Variations in the opposite direction, tending to thwart the adaptive modification, will be disadvantageous and will be eliminated. ”

“ The organism, so far as variation is concerned, is a complex aggregate of such pendulums. Suppose then that it has reached congenital harmony with its environment. The pendulums are all swinging through the small arc implied by the slight variations which occur even among the offspring of the same parents. No pendulum can materially increase its swing; for since the organism has reached congenital harmony with its environment, any marked variation will be out of harmony and the individual in which it occurs will be eliminated. ”

“ For if brain substance, for example, contributes nothing to the reproductive cells, any modification it acquires during individual life can only reach the germ through some indirect mode of influence. But does it-does any modification of the body substance -- so affect the germ as to become hereditary? Prof. Weismann answers this question by asserting that the evidence for the direct transmission of acquired characters is wholly insufficient, and by contending that, until· satisfactory evidence is forthcoming, we may not accept transmission as a factor in evolution. ”

“ This was termed by Romanes 'selection value.' And one of the difficulties which critics of natural selection have felt is that the little more or the little less of variation must often be too small in amount to be of selection value so as to determine survival. ”

“ Darwin accepted such transmission as subordinate to natural selection, and attempted to account for it by his theory of pangenesis. According to that hypothesis all the component cells of an organism throw off minute gemmules, and these and their like, collecting in the reproductive cells, are the parental germs from which all the cells of the offspring of that organism are developed. ”

“ Weismann. According to his conception, variations of germinal origin occur from time to time. By its innate plasticity the several parts of an organism implicated by their association with the varying part are modified in individual life in such away that their modifications cooperate with the germinal variation in producing an adaption of double origin, partly congenital, partly acquired. The organism then waits, so to speak, for a further congenital variation, when a like process of adaptation again occurs ”

“ If the bone is broken and heals out of the straight, the plates of the spongy tissue become rearranged so as to be in the new direction of greatest tension and pressure; thus they can adapt themselves to changed circumstances. ”

“ 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. ”