THE OUTER AND INNER TISSUES OF ANIMALS. 287 



on the organism cannot fail to change its surface more 

 than its centre, and so differentiate the two ; while on the 

 other hand, the surfaces of organisms inhabiting the same 

 medium display extreme unlikenesses which cannot be due to 

 the immediate actions of their medium. Let us dwell a 

 moment on the antithesis. 



We have abundant evidence that animal protoplasm is 

 rapidly modified by light, heat, air, water, and the salts 

 contained in water — coagulated, turned from soluble into in- 

 soluble, partially changed into isomeric compounds, or other- 

 wise chemically altered. Immediate metamorphoses of this 

 kind are often obviously produced in ova by changes of their 

 media. At the outset, therefore, before yet there existed 

 any such differentiation as that which now usually arises by 

 inheritance, these environing agencies must have tended to 

 originate a protective envelope. For a modification produced 

 by them on the superficial part of the protoplasm, must 

 either have been a decomposition or else the formation of a 

 compound that remained stable under their subsequent action. 

 There would be generated an outer layer of substance that 

 was so molecularty immobile as to be incapable of farther 

 metamorphoses, while it would shield the contained proto- 

 plasm from that too great action of external forces which, by 

 rapidly changing the unstable equilibrium of its molecules 

 into a relatively stable equilibrium, would arrest development. 

 Evidently organic evolution, whether individual or general, 

 must always and everywhere have been subordinate to these 

 physical necessities. Though natural selection, beginning 

 with minute portions of protoplasm, must all along have 

 tended to establish a molecular composition apt to undergo 

 this differentiation of surface from centre to the most favour- 

 able extent ; yet it must all along have done so while con- 

 trolled by this process of direct equilibration. 



Contrariwise, the many and great unlikenesses among the 

 dermal structures of creatures inhabiting the same element, 

 cannot be ascribed to any such cause. The contrasts between 

 Yol. II. 13 



