^'-^^ THE EVOLUTION OF LIFE. 



the heterogeneity increases, the complications arising from 

 this multiplication of effects grow more marked ; since 

 the more strongly contrasted the parts of an aggregate 

 become, the more different must be their reactions upon 

 incident forces, and the more unlike must be the secondary 

 sets of effects which these modified incident forces initiate ; 

 and since every increase in the number of unlike parts 

 increases the number of such differentiated incident forces, 

 and such secondary sets of effects. 



How this multiplication of effects conspires with the in- 

 stability of the homogeneous, to work an increasing multi- 

 formity of structiu'e in an organism, was shown at the time ; 

 and the foregoing pages contain further incidental iQustra- 

 tions. Under the head of Adaptation (§ 69), it was shown 

 that a change in one function must act and re-act through 

 ever-compHcating perturbations on the rest; and that, eventu- 

 ally, all parts of the organism must be modified in their 

 states. Suppose that the head of a mammal becomes very 

 mu.ch more weighty — what must be the indirect results ? 

 The muscles of the neck are put to greater exertions ; and 

 its vertebrae have to bear additional tensions and pressures, 

 caused both by the increased weight of the head, and the 

 stronger contractions of the muscles that support and move the 

 head. These muscles also affect their own attachments : several 

 of the dorsal spines have augmented strains put on them ; 

 and the vertebras to which they are fixed, are more severely 

 taxed. Further, this heavier head and the more massive 

 neck it necessitates, require a stronger fulcrum : the whole 

 thoracic arch, and the fore limbs which support it, are sub- 

 ject to greater continuous stress and more violent occasional 

 shocks. And the required strengthening of the fore-quarters 

 cannot take place, without the centre of gravity being 

 changed, and the hind limbs being differently reacted upon 

 during locomotion. Any one who compares the outline 

 of the bison with that of its congener, the ox, will clearly 

 see how profoundly a heavier head affects the entire osseous 



