536 THE EVOLUTION OF LIFE. 



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 constitution by going wrong some in one way 

 iand some in another; 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 balance among the powers of the 

 organs composing an organism, yet their 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 economise in 

 every part o^ 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 

 are required, or if a bone is stronger than needful, no advant- 

 age results but rather a disadvantage — a disadvantage which 

 will decrease the chances of survival. Hence it 



follows that among any organs which habitually act in con- 

 cert, 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 muscles and 

 ligaments to secure the neighbouring articulations; must 

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

 more massive nerve to transmit stimulus, and some extra 



