DIRECT EQUILIBRATION. 527 



have the required thickness ? or, indeed, how could it come to 

 exist at all? Suppose this protective envelope to be too 

 weak, so that some of the eggs a bird lays are broken or 

 cracked. In the first place, the breakages or crackings are 

 actions which cannot react on the maternal organism in such 

 ways as to cause the secretion of thicker shells for the 

 future: to suppose that they can, is to suppose that the 

 bird understands the cause of the evil, and that the secre- 

 tion of thicker shells can be effected by its will. In the 

 second place, such developing chicks as are contained in the 

 shells which crack or break, are almost certain to die; and 

 cannot, therefore, acquire appropriately-modified constitu- 

 tions : even supposing any relation could exist between the 

 impression received and the change required. Meanwhile, 

 such eggs as escape breakage are not influenced at all by the 

 requirement; and hence, on the birds developed from them, 

 there cannot have acted any force tending to work the need- 

 ful adjustment of functions. In no way, therefore, can a 

 direct equilibration between constitution and conditions be 

 here produced. Even in organs that can be modified 



by certain incident actions into correspondence with such in- 

 cident actions, there are some re-adjustments which cannot 

 be effected by direct balancing. It is thus with the bones. 

 The majority of the bones have to resist muscular strains; 

 and variations in the muscular strains call forth, by reaction, 

 variations in the strengths of the bones. Here there is 

 direct equilibration. But though the greater massiveness 

 'cquired by bones subject to greater strains, may be ascribed 

 3 counter-acting forces evoked by forces brought into 

 action; it is impossible that the acquirement of greater 

 lengths by bones can be thus accounted for. It has been 

 supposed that the elongation of the metatarsals in wading 

 birds, has resulted from direct adaptation to conditions of 

 life. To justify this supposition, however, it must be shown 

 that the mechanical actions and reactions in the legs of a 

 wading bird, differ from those in the legs of other birds ; and 

 that the differential actions are equilibrated by the extra 



