ORIGIN OF VARIATIONS. 167 



the organism during the period in which the devel- 

 opment of the parts, and the greatest amount of 

 growth takes place. In the growing and develop- 

 ing animal there is a constant and rapid physical 

 change taking place, a process of building up which 

 may be easily modified. In the adult the physical 

 change is not so great ; the ordinary restoration 

 does not exceed the waste and wear, and any pro- 

 nounced change in the forces acting on the organism 

 is likely to interfere and decrease the restoration 

 without causing growth in any new direction. This 

 seems probably due to the fact that in the adult 

 the nervous associations are stronger, and co-ordi- 

 nations once broken are not easily replaced by 

 new ones. 



The phenomena of acclimatisation illustrate this 

 principle. One cannot read the passage relating 

 to this subject in Darwin's Plants and Animals 

 under Domestication without being convinced that 

 it is the action of the changed environment upon 

 the young and developing organisms which produces 

 the changes of constitution which are denoted by 

 acclimatisation. The fact that acclimatisation does 

 not take place in plants propagated by buds and 

 grafting seems to show that, in order that the 

 changed forces may produce their greatest effect, 

 they must act upon the organism during the period 



