THE OUTER TISSUES OF ANIMALS. 297 



the modification and the accompanying habit, the thickening 

 would be further increased in the same way — survival of the 

 fittest tending ever to accelerate the process. Presently the 

 horny nodes so formed, hitherto defensive only in their 

 effects, would, by their prominence, become offensive — would 

 make the blows given more hurtful. And now natural 

 selection, aiding more actively, would mould the nodes into 

 spurs : the individuals in which the nodes were most pointed 

 would be apt to survive and propagate ; and the pointedness 

 generation after generation thus increased, would end in the 

 well- adapted shape we see. 



But if in these cases the differentiations which fit particular 

 parts of the outer tissues to bear rough usage, are caused 

 mainly by the direct balancing of external actions by in- 

 ternal reactions, then we may suspect that the like is true 

 of other modifications that occur where special strains and 

 abrasions have to be met. Possibly it is true of sundry parts 

 that are formed of hardened epidermis, such as the nails, 

 claws, hoofs, and hollow horns of Mammals ; "all of which," 

 says Prof. Huxley, " are constructed on essentially the same 

 plan, being diverticula of the whole integument, the outer 

 layer of whose ecderon has undergone horny metamorphosis." 

 Leaving open, however, the question what tegumentary 

 structures are due to direct equilibration, furthered and con- 

 trolled by indirect equilibration, it is tolerably clear that 

 direct equilibration has been one of the factors. 



How has it produced its effects ? that is to say — by what 

 physical processes do pressure and friction bring about dermal 

 hardenings ? To this inquiry there is an answer similar to 

 that which was given to the inquiry respecting the formation 

 of wood. (§ 280-2.) As in plants we saw that intermittent 

 compressions of sap- canals increase the exudation of sap, and 

 thus cause increased deposits of its contained substances in 

 the surrounding tissues ; so in animals, we have good reason 

 for concluding that intermittent compressions of the capil- 

 laries increase the exudation of serum, and by thus supplying 



