THE OUTER TISSUES OF ANIMALS. 295 



these tissues may be expected to grow : the area supplied by 

 the increased currents of blood set up by this exchange, will 

 become protuberant will bud out ; and the bud so forme.1 

 will give origin to secondary buds at those parts of its surface 

 which, as before, are most favourably circumstanced for 

 carrying on the aeration. Of course this process will be 

 checked where, though otherwise advantageously placed, the 

 growing branchiae would be specially liable to damage, or 

 would be great hindrances to the creature's movements. But 

 bearing in mind that functionally-produced adaptation will 

 here, as in other cases, be both aided and controlled by 

 natural selection, we may ascribe to it an important share, if 

 not a leading share, in the differentiation. 



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 ? Taking, for instance, the cal- 

 losities 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 ; since it 

 implies 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 



