450 THE EVOLUTION OF LIFE. 



of natural selection, we know that there exists a balance 

 among the powers of organs which habitually act together 

 such proportions among them, that no one has any consider- 

 able excess of efficiency. We see, for example, that through- 

 out the vascular system, there is maintained an. equilibrium 

 between the powers, that is, the developments, of the com- 

 ponent parts : in some cases, under excessive exertion, the 

 heart gives way, and we have enlargement ; in other cases 

 the large arteries give way, and we have aneurisms ; in other 

 cases the minute blood-vessels give way now bursting, now 

 becoming chronically congested. That is to say, in the 

 average constitution, no superfluous strength is possessed 

 by any of the appliances for circulating the blood. Take, 

 again, a set of motor organs. Great strain here causes the 

 fibres of a muscle to tear. There the muscle does not yield 

 but the tendon snaps. Elsewhere neither muscle nor tendon 

 is damaged, but the bone breaks. Joining with these instances 

 the general fact, that under the same adverse conditions, 

 different individuals show their slight differences of consti- 

 tution by going wrong some in one way and some in an- 

 other ; and that even in the same individual, similar adverse 

 conditions will now affect one viscus and now another ; it 

 becomes manifest that though there cannot be maintained 

 an accurate or absolute balance among the powers of the 

 organs composing an organism, yet the excesses and de- 

 ficiencies of power are extremely slight. That they must be 

 extremely slight, is, as before said, a deduction from the 

 hypothesis of natural selection. Mr Darwin himself argues 

 " that natural selection is continually trying to economize in 

 every part of the organization. If under changed conditions 

 of life a structure before useful becomes less useful, any 

 diminution, however slight, in its development, will be seized 

 on by natural selection, for it will profit the individual not 

 to have its nutriment wasted in building up an useless struc- 

 ture." In other words, if any muscle has more fibres than 

 can be utilized, or if a bone be stronger than needful, no ad- 



