312 PHYSIOLOGICAL DEVELOPMENT. 



ment somewhat more developed, seems here the only likely 

 cause. 



§ 293. Among the conspicuous modifications by which the 

 originally-uniform outer layer is rendered multiform, are the 

 protective structures. Let us look first at the few cases in 

 which the formation of these is ascribable mainly to direct 

 equilibration. 



Already reference has been more than once made to those 

 thickenings that occur where the skin is exposed to unusual 

 pressure and friction. Are these adaptations inheritable? 

 and may they, by accumulation through many generations, 

 produce permanent dermal structures fitted to permanent or 

 frequently-recurring stress? Take, for instance, the callosi- 

 ties on the knuckles of the Gorilla, which are adapted to 

 its habit of partially supporting itself on its closed hands 

 when moving along the ground. Shall we suppose that these 

 defensive thickenings are produced afresh in each individual 

 by the direct actions; or that they are inherited modifica- 

 tions caused by such direct actions; or that they are wholly 

 due to the natural selection of spontaneous variations ? The 

 last supposition does not seem a probable one. Such thicken- 

 ings, if spontaneous, would be no more likely to occur on the 

 knuckles than on any other of the hundred equal areas form- 

 ing the skin-surface at large; and the chances against their 

 simultaneous occurrence on all eight knuckles would be in- 

 calculable. Moreover, the implication would be that those 

 slight extra thicknesses of skin on the knuckles, with which 

 we must suppose the selection to have commenced, were so 

 advantageous as to cause survivals of the individuals having 

 them, in presence of other superiorities possessed by other 

 individuals. Then that survivals so caused, if they ever 

 occurred at all, should have occurred with the frequency 

 requisite to establish and increase the variation, is hardly 

 supposable. And if we reject, as also unlikely, the repro- 

 duction of these callosities de novo in each individual (for 



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