If the regulation of the strength of the integumentary skeleton be referred to selection, we see at once why carapace and wing-covers should be of equal thickness throughout their whole extent, and why they do not disappear, although they do not function actively, and are less stimulated than any other parts of the skeleton; and we also understand why the abdomen of hermit-crabs and of larval Phryganidæ and Psychidæ has become soft, whether it be exposed to pressure or friction in a greater or a less degree. It no longer requires to be hard, because it is protected by the house, and in the case of the Pagurids it must not be hard because it could not then be readily squeezed into the hard-walled and narrow recesses of the Gasteropod shell; in this case there has therefore been positive selection. I have not yet referred to the fact that the chitinous covering is certainly not living, though it is not exactly dead; it is a secretion of the epidermic cells, not a tissue, and we cannot suppose that there are any nerve-endings in it. It is almost superfluous to say that the fact that the skin is cast is in itself enough to make such an assumption untenable, for the whole of the assumed delicate nervous network would be shed at every moult and torn away from the nerves which lead to it. As far as my knowledge goes, nothing of this kind occurs anywhere in the whole range of the animal kingdom. Even if we assume, for the benefit of Zehnder's hypothesis, that although there are no nerves in the chitin itself yet irritations affecting the chitinous coat may be transmitted through it to the delicate nerve-endings lying beneath it, this should take place in a greater degree at the thin places of the skeleton than _at the thick parts_! But this interpretation is again fallacious, for we see that the tactile organs of Arthropods always break through the chitinous cuticle and protrude beyond it in the form of setæ. Of the many other opponents of my views in regard to the transmissibility of acquired functional modifications, I need only deal in detail with Oscar Hertwig. He seeks for direct proofs of an inheritance of acquired characters, and believes that he has found these in the hereditary transmission of acquired immunity from certain diseases. He reminds us of Ehrlich's well-known experiments on mice with ricin and abrin. Even small doses of these two poisons kill mice, but they are tolerated in very minute doses, and if their administration be continued for some time in such minute doses, the animals gradually acquire a high degree of insensitiveness to these poisons; they become immune to ricin and abrin.