DIRECT EQUILIBRATION. 525 



changes may be directly caused by changes in the environ- 

 ment. The generative apparatus contained in every flower 

 acts only once during its existence; and even then, the parts 

 subserve their ends in a passive rather than an active way. 

 Functionally-produced modifications are therefore out of the 

 question. If a plant's anthers are so placed that the insect 

 which most commonly frequents its flowers, must come in 

 contact with the pollen, and fertilize with it other flowers of 

 the same species; and if this insect, dwindling away or dis- 

 appearing from the locality, leaves behind no insects having 

 such shapes and habits as cause them to do the same thing 

 efficiently, but only some which do it inefficiently; it is 

 clear that this change of its conditions has no immediate 

 tendency to work in 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 that some other insect could serve them as pollen- 

 carrier, would have good chances of perpetuating themselves. 

 And on their progeny, inheriting the deviation, there would 

 act no external force directly tending to make the deviation 

 greater ; 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 equilibration 

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

 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 

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



