232 THE INDUCTIOiSrS OF BIOLOGY. 



tive breeding, one variety of horse has had its locomotive 

 power increased considerably beyond the locomotive powers 

 of other varieties; yet further increase takes place, if at all, 

 at an inappreciable rate. The different kinds of dogs, too, in 

 which different forms and capacities have been established, 

 do not now show aptitudes for diverging in the same direc- 

 tions at 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 con- 

 sequent margin for 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 indi- 

 vidual and in a race; but tliat 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 also have 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 sizes in these members, begins to lose such extra 

 sizes on ceasing to exert the members ; and eventually lapses 

 more or less nearly into the original state. Legs strength- 

 ened by a pedestrian tour, become relatively weak again after 

 a prolonged return to sedentary life. The acquired ability to 

 perform feats of skill disappears in course of time, if the per- 

 formance of them be given up. For comparative failure in 

 executing a piece of music, in playing a game at chess, or in 

 anything requiring special culture, the being out of practice 



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

 argument have obliged me to forestall myself, by assuming the conclusion 

 reached in a subsequent chapter, that modifications of structure produced by 

 modifications of function arc transmitted to offspring. 



