306 PHYSIOLOGICAL DEVELOPMENT. 



with spines, which protects the Diodon or the Cyclichthys 

 from enemies it could not escape, comes within the general 

 formula of an outer tissue differentiated from inner tissues 

 by the outer actions to which the creature is exposed: the 

 differentiation having gone on until there is equilibrium 

 between the destructive forces to be met and the protective 

 forces which meet them. 



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 con- 

 verse with the agencies in the environment, most of them 

 organic, we may assign to the constant and universal con- 

 verse 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 repro- 

 duced. 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 



