HEREDITY. 



245 



for several reasons, comparatively difficult. Changes pro- 

 duced in the sizes of parts by changes in their amounts of 

 action, are mostly unobtrusive. A muscle that has increased 

 in bulk, is so obscured by natural or artificial clothing, that un- 

 less the alteration is extreme it passes without remark. Such 

 nervous developments as are possible in the course of a single 

 life, cannot be seen externally. Visceral modifications of a 

 normal kind, are observable but obscurely, or not at all. And 

 if the changes of structure worked in individuals by changes 

 in their habits, are thus difficult to trace ; still more difficult 

 to trace must be the transmission of them — further hidden, 

 as this is, by the infl.uence of other individuals that are often 

 otherwise modified by other habits. Moreover, such special- 

 ities of structure as are due to specialities of function, are 

 usually entangled with specialities of structure that are, or 

 may be, due to selection, natural or artificial. In the majority 

 of cases, it is impossible to say that a structural peculiarity 

 which seems to have arisen in offspring from a functional 

 peculiarity in the parent, is wholly independent of some 

 congenital peculiarity of structure in the parent, which in- 

 duced this functional peculiarity. We are restricted to 

 cases with which natural or artificial selection can have had 

 nothing to do ; and such cases are difficult to find. Some, 

 however, may here be noted. 



A species of plant that has been transferred from one soil 

 or climate to another, frequently undergoes what botanists 

 call '* a change of habit '^ — a change which, without affecting 

 its specific characters, is yet conspicuous. In its new locality, 

 the species is distinguished by leaves that are much larger, 

 or much smaller, or differently shaped, or more fleshy ; of 

 instead of being, as before, comparatively smooth, it becomes 

 hairy; or its stem becomes woody instead of being herbaceous; 

 or its branches, no longer growing upwards, assume a droop- 

 ing character. Now these '^ changes of habit'' are clearly de- 

 termined by functional changes. Occurring, as they do, in 

 many individuals that have undergone the same transportation, 



