70 EVOLUTION AND NATURAL THEOLOGY. 



rative power is always narrowly limited) and this 

 involves the decay and death of the organism. 

 The exercise of any organ or faculty is always 

 beneficial within certain limits marked out by 

 the potentialities of the organism itself. Al- 

 though these limits cannot be exceeded, they are 

 capable of gradual improvement within their 

 possibilities of extension. Thus, the eye might 

 be gradually trained to bear a light which it 

 could not possibly endure at first ; or a load may 

 be carried, after repeated trials, greater than 

 could be lifted at the first attempt. But even as 

 the slightesf motion involves a certain drain on 

 the vital resources of an organism, so an im- 

 provement in any organ implies a demand upon 

 the organism which must be met in one form or 

 another. An animal may excel in either strength 

 or speed, but any abnormal increase in one 

 quality would generally be at the expense of the 

 other. We cannot combine the speed of the 

 race-horse, and the strength of the dray-horse in 

 the same animal. Therefore, although organs or 

 faculties are strengthened by exercise, it is also 

 true that any which are not much used become 

 more or less weakened by disuse, the unused 

 -organs receiving less than their proper share of 



