308 PHYSIOLOGICAL DEVELOPMENT. 



tinuity. That is to say, these literally outer and qium-outet 

 layers are capable of rapidly assuming one another s struc 

 tures and functions when subject to one another s conditions. 

 Mucous surfaces, normally kept covered, become skin -like if 

 exposed to the air ; but resume more or less fully their 

 normal characters when restored to their normal positions. 

 These are truths familiar to pathologists. They continually 

 meet with proofs that permanent evcrsion of the mucous 

 membrane, even where it- is by prolapse of a part deeply 

 seated within the body, is followed by an adaptation eventu 

 ally almost complete : originally moist, tender to the touch, 

 and irritated by the air, the surface gradually becomes 

 covered with a thick, dry cuticle ; and is then scarcely more 

 sensitive than ordinary integument. 



Whether this equilibration between new outer forces and 

 reactive inner forces, which is thus directly produced in in 

 dividuals, is similarly produced in races, must remain as a 

 question not to be answered in a positive way. On the one 

 hand, we have the fact that among the higher animals there 

 are cases of quasi-outer tissues which are in one species 

 habitually ensheathed, while in another species they are not 

 ensheathed ; and that these two tissues, though unquestion 

 ably homologous, differ as much as skin and mucous mem 

 brane differ. On the other hand, there are certain analogous 

 changes of surface, as on the abdomen of the Hermit-Crab, 

 which give warrant to the supposition that survival of the 

 fittest is the chief agent in establishing such differentiations ; 

 since the abdomen of a Hermit-Crab, bathed by water within 

 the shell it occupies, is not exposed to physical conditions 

 that directly tend to differentiate its surface from the surface 

 of the thorax. But though in cases like this last, we must 

 assign the result to the natural selection of variations arising 

 incidentally ; \ve may I think legitimately assign the result 

 to the immediate action of changed conditions where, as in 

 cases like the first, we see these producing in the individual, 

 effects of the kinds observed in the race. 



