INDIRECT EQUILIBRATION 537 



development of a nervous centre to supply the extra stimu 

 lus. The question arises, then, do variations of the appro 

 priate kinds occur simultaneously in all these co-operative 

 parts? Have we any reason to think that the parts spon 

 taneously increase or decrease together? The assumption 

 that they do seems to me untenable; and its untenability 

 will, I think, become conspicuous if we take a case, and 

 observe how .extremely numerous and involved are the varia 

 tions which must be supposed to occur together. In 

 illustration of another point, we have already considered the 

 modifications required to accompany increased weight of the 

 head (155). Instead of the bison, the moose deer, or the 

 extinct Irish elk, will here best serve our purpose. In this 

 last species the male has enormously-developed horns, used 

 for purposes of offence and defence. These horns, weighing 

 upwards of a hundred-weight, are carried at great mechanical 

 disadvantage : supported as they are, along with the massive 

 skull which bears them, at the extremity of the outstretched 

 neck. Further, that these heavy horns may be of use in 

 fighting, the supporting bones and muscles must be strong 

 enough, not simply to carry them, but to put them in motion 

 with the rapidity needed for giving blows. Let us, then, ask 

 how, by natural selection, this complex apparatus of bones 

 and muscles can have been developed, pari passu with the 

 horns? If we suppose the horns to have been originally of 

 like size with those borne by other kinds of deer; and if we 

 suppose that in some individual they became larger by 

 spontaneous variation; what would be the concomitant 

 changes required to render their greater size useful? Other 

 things equal, the blow given by a larger horn would be a 

 blow given by a heavier mass moving at a smaller velocity: 

 the momentum would be the same as before; and the area 

 of contact with the body struck being somewhat increased, 

 while the velocity was decreased, the injury done would be 

 less. That horns may become better weapons, the whole 

 apparatus concerned in moving them must be so strength- 



