512 THE EVOLUTION OF LIFE. 



an aggregate become, the more different must.be their reac 

 tions on incident forces, and the more unlike must be the 

 secondary effects which these initiate; and because every 

 increase in the number of unlike parts adds to the number 

 of such differentiated incident forces, and such secondary 

 effects. 



How this multiplication of effects conspires, with the in 

 stability of the homogeneous, to work an increasing multi 

 formity of structure in an organism, was shown at the time; 

 and the foregoing pages contain further incidental illustra 

 tions. In 69 it was pointed out that a change in one 

 function must produce ever-complicating perturbations in 

 other functions; and that, eventually, all parts of the 

 organism must be modified in their states. Suppose that the 

 head of a bison becomes much heavier, what must be the 

 indirect results ? The muscles of the neck are put to greater 

 exertions; and its vertebra have to bear additional tensions 

 and pressures, caused both by the increased weight of the 

 head, and by the stronger contractions of the muscles that 

 support and move it. These muscles also affect their special 

 attachments: several of the dorsal spines suffer augmented 

 strains; and the vertebra? 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 subject 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 out 

 line of the bison with that of its congener, the ox, will see 

 how profoundly a heavier head affects the entire osseous 

 and muscular systems. Besides this multiplica 



tion of mechanical effects, there is a multiplication of 

 physiological effects. The vascular apparatus is modified 

 throughout its whole structure by each considerable modifi- 



