HEREDITY. 307 



consumption, and insanity is universally admitted. Among 

 the less-common diseases of which the descent has been ob- 

 served, are ichthyosis, leprosy, pityriasis, sebaceous tumours, 

 plica polonica, dipsomania, somnambulism, catalepsy, epi- 

 lepsy, asthma, apoplexy, elephantiasis. General nervousness 

 displayed by parents almost always re-appears in their chil- 

 dren. Even a bias towards suicide appears to be sometimes 

 hereditary. 



§ 82. To prove the transmission of those structural pecu- 

 liarities which have resulted from functional peculiarities, is, 

 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 which has increased 

 in bulk is usually so obscured by natural or artificial cloth- 

 ing, that unless 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 obscurelv, 

 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 influences of other 

 individuals who are often otherwise modified by other 

 habits. Moreover, such specialities of structure as are due 

 to specialities of function, are usually entangled with speciali- 

 ties of structure which are, or may be, due to selection, natural 

 or artificial. In most cases it is impossible to say that a 

 structural peculiarity which seems to have arisen in offspring 

 from a functional peculiarity in a parent, is wholly inde- 

 pendent of some congenital peculiarity of structure in the 

 parent, whence this functional peculiarity arose. 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 be noted. 



A species of plant that has been transferred from one soil 



