THE INNER TISSUES OF ANIMALS. 845 



So that both by the increased quantity of dense matter and 

 by its mechanically more- advantageous position, the bendiiigs 

 of the bone are progressively decreased. But as they ara 

 decreased, each new layer formed on the concave surface, has 

 its substance and its capillaries less compressed; and the 

 resulting growth and induration are rendered less rapid. 

 Evidently, then, the additions, slowly diminishing, will 

 eventually cease ; and this will happen when the bone no 

 longer bends. That is to sa}'', the thickening of the bone will 

 reach its limit when there is equilibrium between the inci- 

 dent forces and the forces which resist them. Here, indeed, 

 we may trace with great clearness the process of direct 

 equilibration — may see how an unusual force, falling on the 

 moving equilibrium of an organism and not overthrowing it, 

 goes OR working modifications until the re-action balances 

 the action. 



That, however, which now chiefly concerns us, is to note 

 how this marked adaptation supports the general argument. 

 Unquestionably bone is in this case formed under the influ- 

 ence of mechanical stress, and formed just where it most 

 efiectually meets the stress. This result, not otherwise 

 explained, is explained by the hypothesis above set forth. 

 And when we see that this special deposit of bone is ac- 

 counted for by actions like those to which bone-formation in 

 general is ascribed, the probability that these are the actions 

 at work becomes very great. 



Of course it is not alleged that osseous structures arise in 

 this way alone. The bones of the skull and various dermal 

 bones cannot be thus interpreted. Here the natural selec- 

 tion of favourable variations appears the only assignable 

 caua^ — the equilibration is indirect. We know that ossific 

 deposits now and then occur in tissues where they are not 

 usually found ; and such deposits, originally abnormal, if 

 they occurred in places where advantages arose from them, 

 might readily be established and increased by survival of the 

 fittest. Especially might we expect this to happen when a 



