When we admit that the fit adaptation of the organism, as far as we understand it, must depend upon processes of selection, we may refer this 'functional adaptation' also to primitive processes of selection, which prevailed at the very beginning of life upon our earth, and represented, so to speak, the first adaptation that was established, but we can say nothing with certainty in regard to this matter as long as we do not understand the essence of assimilation. It is conceivable, however, that a primary adaptiveness may have arisen, so to speak, abruptly, through a concurrence of favourable circumstances, as we shall endeavour to show later on when we discuss the beginnings of life.
Even although we cannot lay bare the primary roots of 'functional adaptation' we can gain from the fact itself very valuable insight into phenomena which would otherwise be unintelligible and mysterious: the perfectly adapted structure of many tissues and their power of adaptation to changed conditions. In this lies, in the main, the advance in our knowledge which is due to Roux's Kampf der Theile.
If a number of embryonic cells of different capacity, say A, B, and C, be affected by different kinds of functional stimuli, a, b, and c, those cells will grow most rapidly which are most frequently affected by the stimulus appropriate to them. The proportion in which the cells A, B, and C will ultimately be present in the tissues will depend upon the frequency with which the stimuli a, b, and c act upon the tissue. But the tissue will be still more precisely determined as to its structure if the three kinds of stimuli affect the cell-mass, not uniformly all over, but only at certain spots, or along particular paths, one in this, the other in that. Thus the cells A will predominate over the cells B and C at all the places which are most frequently affected by the stimulus a, the cells B in the sphere of the stimulus b, and the cells C in that of the stimulus c; there they will increase most rapidly and so crowd out the other kinds of cells, and thus a spatial arrangement will be established within the tissue, a 'structure' which corresponds and is well adapted to its end. This is what Roux deduced from his Struggle of the Parts, and I subsequently defined the process as histonal or tissue selection.
Let us first take an example. The anatomist Hermann Meyer showed in 1869 that the so-called 'spongiosa,' that is, the bony tissue of spongy structure within the terminal portions of the long bones in Man and Mammals, has a minute structure conspicuously well adapted to its office. The thin bone lamellæ of this 'spongiosa' lie precisely in the direction of the strongest strain or pressure which is exerted upon the bone at the particular area. Arch-like in form, they are kept apart by means of buttresses, and no architect could have done better if he had been entrusted with the task of making a complicated system of arches with the greatest possible carrying and resisting power combined with the greatest possible economy of material.
This well-adapted structure is now interpreted through the Struggle of the Parts as a self-differentiation, for if there be in the rudiments or primordia of the bone differently endowed elements[10], that is, cells which respond in diverse ways to different stimuli, these must arrange themselves locally, owing to the struggle for space and food, in a manner corresponding to the distribution of the different stimuli in the bone. The largest amount of bone substance will be formed in the directions of the strongest strain and the greatest pressure, because the bone-forming cells are excited by this, their functional stimulus, to growth and multiplication. Thus the buttress and arch structure comes about, and between the delicate bone lamellæ spaces will remain free, and these, being relieved from the burden of strain and pressure by the aforesaid bony lamellæ, will offer suitable conditions of life to cells with other functional properties, such as connective tissue cells or vascular cells.
[10] I do not here enter into the question whether we have not in this case to do with similar elements, which have the power of differentiating into one or another kind of cell according to the nature of the external stimuli by which they are influenced.
The structure of the bone spongiosa is not everywhere the same, and it is demonstrably related with precision to the conditions of strain and pressure at each particular region. Thus, just below the soft cartilaginous covering of the joints there are no long pillars with short arches, but only rounded meshes, because the pressure is here almost equally strong from all sides. The long parallel pillars only occur further down in the bone, and they lie in two directions which intersect each other obliquely, corresponding to the two main directions of pressure. But it is only under the functional stimulus of pressure that the bone-forming cells have an advantage over the others, and multiply more quickly, thus crowding out those that are not attuned to the appropriate functional stimulus.
In a similar manner Roux interprets, in the light of the struggle of the parts, the striking adaptations in the course, the branching, and the lumen-formation of the blood-vessels, in the direction of the intersecting connective tissue strands in the tail-fin of the dolphin, in the direction of the fibres in the tympanum, and in many other adaptations in the histological structure of complex tissues.
In this there is manifestly an important step of progress, for it is obvious that the direction of the bone-lamellæ and such like could not have been determined by individual selection, and the same is true in regard to many other histological details. It cannot be disputed, however, that there is a kind of selection-process here also, similar to that which we think of, with Darwin and Wallace, as occurring between individual organisms. Just as in the latter, which we shall henceforward call personal selection, variability and inheritance lead, in the struggle for existence, to the survival of the fittest, so, in histonal differentiation, the same three factors lead to the victory of what is best suited to the parts of the body in question. The tissues and the parts of the tissues have to distribute and arrange themselves so that each comes to fill the place in which it is most effectively and frequently affected by its specific stimulus, that is, the stimulus in regard to which it is superior to other parts; but these places are also those the occupation of which by the best re-acting parts makes the whole tissue capable of more effective function, and therefore makes its structure the fittest. Variability—in this case that of embryonic cells, with different primary constituents—must be assumed; inheritance is implied in the multiplication of the cells by division; and the 'struggle for existence' here assumes its frequent form of a competition for food and space; the cells which assimilate more rapidly because of the more frequent functional stimulus increase more rapidly, draw away nourishment from the more slowly-multiplying cells around them, and thus crowd these out to a greater or less extent.
We might even speak of histonal selection among unicellulars, for it is conceivable that in primitive living substance, such as that of a moneron, there may be minute differences among the vital particles, involving also functional distinctions, which, under the influence of diverse stimuli, may gradually give rise to an increasingly complex differentiation. For the variations in the primary living substance most strongly affected by a particular stimulus would tend to accumulate at the places most frequently reached by that stimulus, and would crowd out other variations at that spot, just as the body and its individual parts may be said to have taken their architectural form in exact response to the demands made upon them by function. In this case, of course, personal selection and histonal selection co-operate, for every improvement in the organization of the fundamental living substance means at the same time a lasting improvement in the whole individual.