288 PHYSIOLOGICAL DEVELOPMENT. 



naked and shelled Gastropods, between marine Worms and 

 Crustaceans, between soft-skinned Fish and Fish in armour like 

 the Ptericthys, must have been produced entirely by natural 

 selection. Environing forces are, as before, the ultimate 

 causes ; but the forces are now not so much those exercised 

 by the medium as those exercised by the other inhabitants of 

 the medium ; and they do not act by modifying the surface 

 of the individual, but by killing off individuals whose surfaces 

 are least fitted to the requirements: thus slowly affecting the 

 species. The dermal skeleton bristling with spines, which 

 protects the Diodon or the Cyclicthys from enemies it could 

 not escape, still 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 process which is alike in all 

 animals, mainly to the direct actions of their media ; while 

 we ascribe the multitudinous unlikenesses of the process 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, that universal characteristic 

 of tegumentary structures their development into a double 

 layer separated by undifferentiated substance, from which the 

 outermost grows outwardly and the innermost grows in 

 wardly. 



Here let me add a piece of evidence which strengthens 

 very greatly the general argument, at the same time that it 

 justifies this apportionment. When ulceration has gone deep 



