Page:The Effect of External Influences upon Development.djvu/16

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

Wilhelm Roux has given an explanation of the cause of these wonderfully fine adaptations by applying the principle of selection to the parts of the organism. Just as there is a struggle for survival among the individuals of a species, and the fittest are victorious, so also do even the smallest living particles contend with one another, and those that succeed best in securing food and place grow and multiply rapidly, and so displace those that are less suitably equipped. The three factors in the process of selection—variability, heredity, and struggle for existence—are all present. Processes of selection must thus take place amongst every kind of units within the organism,—not only in cells and tissues, but also in the smallest conceivable living particles, which I have called 'biophors.' Everywhere equivalent parts are contending one with another, and everywhere it is the best that prevail. We can describe this process as intra-individual selection, or more briefly, as intra—selection.

It is impossible for me to give an exhaustive account of Roux's argument here, and I must venture to assume that you are familiar with it. But there is one point I must not leave unnoticed: namely that relating to the cause which gives the advantage to one particle over others, and the consequent possibility of a struggle. This cause is to be sought in the relative power of