THE FACTORS OF ORGANIC EVOLUTION. 3 



recognized as the sole process in organic development, the 

 adaptation of parts and powers consequent on the effects of 

 use and disuse that continual moulding and re-moulding of 

 organisms to suit their circumstances, which is brought 

 about by direct converse with such circumstances. 



But while this cause accepted by these few is a true 

 cause, since unquestionably during the . life of the indi 

 vidual organism changes of function produce changes of 

 structure; and while it is a tenable hypothesis that 

 changes of structure so produced are inheritable^; yet it was 

 manifest to those not prepossessed, that this cause cannot 

 with reason be assigned for the greater part of the facts. 

 Though in plants there are some characters which may not 

 irrationally be ascribed to the direct effects of modified 

 functions consequent on modified circumstances, yet the 

 majority of the traits presented by plants are not to be 

 thus explained. It is impossible that the thorns by which 

 a briar is in large measure defended against browsing 

 animals, can have been developed and moulded by the 

 continuous exercise of their protective actions ; for in the 

 first place, the great majority of the thorns are never 

 touched at all, and, in the second place, we have no ground 

 whatever for supposing that those which are touched are 

 thereby made to grow, and to take those shapes which 

 render them efficient. Plants which are rendered uneatable 

 by the thick woolly coatings of their leaves, cannot have 

 had these coatings produced by any process of reaction 

 against the action of enemies; for there is no imaginable 

 reason why, if one part of a plant is eaten, the rest should 

 thereafter begin to develop the hairs on its surface. By 

 what direct effect of function on structure, can the shell of 

 a nut have been evolved ? Or how can those seeds which 

 contain essential oils, rendering them unpalatable to birds, 

 have been made to secrete such essential oils by these 

 actions of birds which they restrain ? QrJ[K&amp;gt;w_ caij_ the 

 delicate plumes borne by some seeds, and giving the wind 



