ADAPTATION. 189 



considerable rates. In domestic animals generally, certain 

 accessions of intelligence have been produced by culture ; but 

 accessions beyond these are inconspicuous. It seems that 

 in each species of organism, there is a margin for functional 

 oscillations on all sides of a mean state, and a consequent 

 margin of structural variations ; that it is possible rapidly to 

 push functional and structural changes towards the extreme 

 of this margin in any direction, both in an individual and 

 in a race ; but that to push these changes further in any 

 direction, and so to alter the organism as to bring its mean 

 state up to the extreme of the margin in that direction, is a 

 comparatively slow process.* 



We have also to note that the limited increase of size pro- 

 duced in any organ by a limited increase of its function, is 

 not maintained unless the increase of function is permanent. 

 A mature man or other animal, led by circumstances into 

 exerting particular members in unusual degrees, and acquir- 

 ing extra- size and power in these members, begins to lose 

 such extra size and power on ceasing to exert these members ; 

 and eventually lapses more or less nearly into the original 

 state. Legs strengthened by a pedestrian tour, become weak 

 again after a prolonged return to sedentary life. The 

 acquired ability to perform feats of skill, disappears in course 

 of time, if the performance of them is given up. For compara- 

 tive failure in executing a piece of music, in playing a game 

 at chess, or in anything requiring special culture, the being 

 out of practice is a reason of which every one recognizes the 

 validity. It is observable, too, that the rapidity and com- 

 pleteness with which an artificial power is lost, is proportionate 

 to the shortness of the cultivation which evoked it. One who 

 has for many years persevered in habits which exercise 

 special muscles or special faculties of mind, retains the extra 



* Here, as in sundry places throughout this chapter, the necessities of the argu- 

 ment have obliged me to forestall myself, by assuming the conclusion reached in a \ 

 subsequent chapter, that modifications of structure produced by modifications of 

 function, are transmitted to offspring. 



