314 PHYSIOLOGICAL DEVELOPMENT. 



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 par- 

 ticular parts of the outer tissues to bear rough usage are 

 caused mainly by the direct balancing of external actions by 

 internal 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. 



§ 294. Dermal structures of another class are developed 

 mainly, if not wholly, by the actions of external causes 

 on species rather than on individuals. These are the 

 various kinds of clothing — hairs, feathers, quills, scales, 

 scutes. Though it is no longer thought as at one time that 

 all these various tegumentary structures are homologous with 

 one another, yet it is unquestionable that sundry of the more 

 conspicuous ones are. Those which are extremely unlike 

 may be seen linked together by a long series of graduated 

 forms. A retrograde metamorphosis from feathers to ap- 

 pendages that are almost scale-like, is well seen in the coat 

 of the Penguin. There is manifest a transition from the 



