DIRECT EQUILIBRATION. 439 



thing efficiently, but only some wliicli do it inefficiently ; it 

 is clear that the change of its concjitions, has no immediate 

 tendency to work ia the plant any such structural change 

 as shall bring about a new balance with its conditions. For 

 the anthers, which, even when they discharge their functions; 

 do it simply by standing in the way of the insect, are, under 

 the supposed circumstances, left untouched by the insect ; 

 and this remaining untouched, cannot have the effect of so 

 modifying the stamens as to bring the anthers into a position 

 to be touched by some other insect. Only those individuals 

 whose parts of fructification so far differed from the average 

 form of the species, that some other insect could serve them 

 as poUen- carrier, would be sufficiently prolific to have good 

 chances of perpetuating themselves. And on their progeny, 

 inheriting the deviation, there would act no external force 

 directly calculated to make the deviation greater, and the 

 adaptation more complete ; since the new circumstances to 

 which re-adaptation is required, are such as do not in the 

 least alter the equilibrium of functions constituting the life 

 of the individual plant. 



§ 162. Among animals, adaptation by direct equiLibrah'on 

 is similarly traceable, wherever, during the life of the uidi- 

 vidual, an external change generates some constant or re- 

 peated change of function. This is conspicuously the case 

 with such parts of an animal as are immediately exposed to 

 diffiised influences, like those of climate, and with such parts 

 of an animal as are occupied in its mechanical actions on the 

 environment. Of the one class of cases, the darkening or 

 b'ghtening of the skin, that follows exposure to greater or 

 less heat, may be taken as an instance; and with tlie other 

 class of cases, we are made familiar by the increase and de- 

 crease which use and disuse cause in tlie organs of motion 

 and manipulation. It is needless here to exemplify these : 

 ihej' were treated of in the Second Part of this work. 



But in animals, as in plants, there are many indispensable 



