FUNCTION. 209 



some cases wholly carried on by them and by the stems. 

 Conversely, the underground parts can partially assume the 

 functions of leaves. The exposed tuber of a potato develops 

 chlorophyll on its surface, and in other cases, as in that of the 

 turnip, roots, properly so called, do the like. In trees the 

 trunks, which have in great measure ceased to produce buds, 

 recommence producing them if the branches are cut off; 

 sometimes aerial branches send down roots to the earth; and 

 under some circumstances the roots, though not in the 

 habit of developing leaf-bearing organs, send up numerous 

 suckers. When the excretion of bile is arrested, part 



goes to the skin and some to the kidneys, which presently 

 suffer under their new task. Various examples of vicarious 

 functions may be found among animals. The excretion of 

 carbonic acid and absorption of oxygen are mainly performed 

 by the lungs, in creatures which have lungs; but in such 

 creatures there continues a certain amount of cutaneous 

 respiration, and in soft-skinned batrachians like the frog, 

 this cutaneous respiration is important. Again, when the 

 kidneys are not discharging their duties a notable quantity 

 of urea is got rid of by perspiration. Other instances 



are supplied by the higher functions. In man the limbs, 

 which among lower vertebrates are almost wholly organs of 

 locomotion, are specialized into organs of locomotion and 

 organs of manipulation. Xevertheless, the human arms and 

 legs do, when needful, fulfil, to some extent, each other's 

 offices. Xot only in childhood and old age are the arms 

 used for purposes of support, but on occasions of emergency, 

 as when mountaineering, they are used by men in full 

 vigour. And that legs are to a considerable degree capable 

 of performing the duties of arms, is proved by the great 

 amount of manipulatory skill reached by them when the 

 arms are absent. Among the perceptions, too, there are ex- 

 amples of partial substitution. The deaf Dr. Kitto described 

 himself as having become excessively sensitive to vibrations 

 propagated through the body; and as so having gained the 



