The foregoing slender contribution to the comparative anatomy and physiology of bursæ is sufficient to show that at certain important and “critical” points in the mammalian anatomy, efficient bursæ are always present. One cannot indeed conceive the function of the parts involved being carried on at all without these ingenious contrivances, and no doubt can exist that in certain of the leading bursæ selection guides and guards, while use and habit maintain them. Over such as these “dominance” or the appearance of mutations might perhaps be supposed to preside, and possibly some useful statistical results might arise from their study from these points of view. But, between these major bursæ in man and lower Primates and the undifferentiated sacs which hardly deserve the name of bursæ, there is a perfect little host of insignificant structures, which at the first attempt at dominion over them on the part of Mendel or de Vries would hoist the standard of revolt. These would even refuse allegiance to Personal Selection under the persuasive banner, “Educability,” which however valuable elsewhere, must stand aside in this little province of Nature. I have thus attempted to “Lyell” this body of facts. Basing the statement on an analysis of a considerable mass of small facts which no one disputes I claim that the modifications drawn from normal anatomy on the one hand and on the other adventitious structures, produced by acknowledged mechanical forces, are examples of the transmission of modifications, and illustrate the mode of formation of certain initial variations. In other regions where Plasto-diēthēsis, as I conceive it, is at work in producing adapted organisms, there may be included in the hyphenated area certain factors of heredity, Mendelian, mutational and others, but not in this group. This is merely an assertion of an opinion though I submit that there is good evidence for it. Not even the hardest hearted Weismannian, Mendelian or mutationist, and not even the biometrician can refuse to this poor little province the required time and mechanical forces, and, unless an opponent can offer some explanation more consistent with the facts than that here offered, the proof of causation is as sound as that shown in the larger one of the direction of hair. CHAPTER XXI. THE PLANTAR ARCH. The principle of Lyell cannot be applied to this section of my subject for it is unique in the animal world. There is here a simple compilation of facts such as the medical schoolboy is supposed to know, and only requires for its setting forth the valuable expert knowledge of our predecessors in anatomy. It is indeed a pedestrian chapter.