DEGENERATION AND LAWS OF VARIATION. 199 



by a change in the environing forces. There is a 

 certain inertia in the nervous co-ordinations. Any 

 single nervous reaction in a connected and oft-re- 

 peated series does not disappear the instant its 

 original stimulus is withdrawn. It will last for 

 some time afterward under the influence of asso- 

 ciation with the rest of the series. Eventually, 

 however, it must disappear, — sooner or later, accord- 

 ing to the firmness of the association. Thus when 

 the forces, acting as stimuli, which have directed the 

 growth of any part, are withdrawn, the power of 

 association will still tend to cause the growth of 

 the part, though less perfectly. If an organ be no 

 longer stimulated to perform its function, it is left 

 without one of the chief directive forces of its 

 growth. The power of association, which at first 

 maintains its normal growth, gradually diminishes ; 

 the inherited impulse weakens, and the organ grad- 

 ually degenerates. Those organs will degenerate 

 most slowly which have existed longest in the race. 

 As examples of this may be mentioned the pineal 

 and thyroid glands among vertebrates ; and these 

 degenerate organs probably owe their persistence 

 partly to the fact that they are associated in growth 

 with such important and unchanging organs as the 

 brain and alimentary canal. 

 The blindness of animals which dwell in caves 



