DIRECT EQUILIBRATION. 441 



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 any appro- 

 priately-modified constitutions : even supposing any con- 

 ceivable relation could be shown, between the impression 

 received and the change required. Meanwhile, such eggs as 

 escape breakage, are not influenced at all by the require- 

 ment ; and hence, on the birds developed from them, there 

 cannot have acted any force tending to work the needful 

 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 forces into correspondence with such 

 incident forces, there are some re-adjustments which cannot 

 be effected by the direct balancing of inner and outer actions. 

 It is thus with the bones. The majority of the bones have 

 to resist muscular strains ; and it is a familiar fact that 

 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 

 acquired by bones subject to greater strains, may be ascribed 

 to a counter-acting force evoked by a force 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 

 lengths. There is not the slightest evidence of this. The 

 metatarsals of a bird, have to bear 110 appreciable strains 

 but those due to the superincumbent weight. Standing in 

 the water does not appreciably alter these strains ; and even 

 if it did, an increase in the lengths of these bones would not 

 fit them any better to meet the altered strains. 



