INDIRECT EQUILIBRATION. 451 



vantage results, but rather a disadvantage — a disadvantage 

 which will decrease the chances of survival. Hence 



it becomes a corollary, that among any organs which habit- 

 ually act in concert, an increase of one can be of no service 

 unless there is a concomitant increase of the rest. The co 

 operative parts must vary together ; otherwise variation will 

 be detrimental. A stronger muscle must have a stronger bone 

 to resist its contractions; must have stronger correlated mus- 

 cles and ligaments to secure the neighbouring articulations ; 

 must have larger blood-vessels to bring it supplies ; must 

 have a more massive nerve to bring it stimulus, and some 

 extra development of a nervous centre to supply this extra 

 stimulus. The question arises, then, — does spontaneous 

 variation occur simidtaneously in all these co-operative 

 parts ? Have we any reason to think that they spontaneously 

 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 ex- 

 tremely numerous and involved are the variations which 

 must be supposed to occur together. In illustration 



of another point, we have already considered the modifica- 

 tion required to accompany increased weight of the head. 

 Instead of the bison, however, the moose deer, or the extinct 

 Irish elk^ will here best serve our purpose. In this species 

 the male has enormously- developed horns, which are used for 

 purposes of offence and defence. These horns, w^eighing up- 

 wards 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 required for giving blows. Let us, 

 then, ask how, by natural selection, this complex apparatus 

 of bones and muscles can have been developed, pari jjassu 

 with the horns ? If w^e suppose the horns to have originally 



