322 PHYSIOLOGICAL DEVELOPMENT. 



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; we 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. 



However this may be, the force of the general argument 

 remains the same. In these exchanges of structure and 

 function between the outer and quasi-outer tissues, we get 

 undeniable proof that they are easily differentiable. And 

 seeing this, we are enabled the more clearly to see how there 

 have, in course of time, arisen those extreme and multi- 

 tudinous differentiations of the outer tissues which have been 

 glanced at. 



