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.