If we venture to apportion the respective shares which mediate and immediate actions have had in differentiating outer from inner tissues, we shall probably not be far wrong in ascribing that part of the result which is alike in all animals, mainly to the direct actions of their media, while we ascribe the multitudinous unlikenesses of the results in various animals, partly to the indirect actions of the media, and partly to the indirect actions of other animals by which the media are inhabited. That is to say, while assigning the specialities of the differentiations to the specialities of converse with the agencies in the environment, most of them organic, we may assign to the constant and universal converse with its inorganic agencies, the universal characteristic of tegumentary structures--their growth outwards from a layer lying below the surface which continually produces new substance to replace the substance worn away or cast off. Here let me add a piece of evidence which strengthens the general argument, at the same time that it justifies this apportionment. When ulceration has gone deep enough to destroy the tegumentary structures, these are never reproduced. The puckered surface formed where an ulcer heals, or where a serious burn has destroyed the skin, consists of modified connective tissue, which, as the healing goes on, spreads inwards from the edges of the ulcer: some of it, perhaps, growing from the portions of connective tissue that dip down between the muscular bundles. This connective tissue is normally covered by the epidermis and thus sheltered from environing actions. What has happened to it? It has now become the outermost layer. And how does it comport itself under its new conditions? It produces a superficial substance which plays the part of the epidermis and grows outwardly. For since the surface, subject to friction and exfoliation, has to be continually renewed, there must be a continual reproduction of an outermost layer from a layer beneath. That is to say, the contact of this deep-seated tissue with outer agencies, produces in it some approach towards that character which we find universally characterizes outer tissue. But while we see under this exposure to the conditions common to all integument, a tendency to assume the structure common to all integument, we see no tendency to assume any of the specialities of tegumentary structure: no rudiments of glands or hair sacs make their appearance.