3 2 E VOL UTION AND DISEASE. 



That increased size is not due always to increased 

 function but may arise from irritation is admirably 

 illustrated in Solatium jasminoides described by Darwin. 

 " The flexible petiole of a half or a quarter grown leaf 

 which has clasped an object for three or four days 

 increases much in thickness, and after several weeks 

 becomes so wonderfully hard and rigid that it can hardly 

 be removed from its support. On comparing a thin 

 transverse slice of such a petiole with one from an 

 older leaf growing close beneath, which has not clasped 

 anything, its diameter was found to be fully doubled 

 and its structure greatly changed " (" Climbing Plants "). 



In this example the extra thickness could not be due 

 to increased function, but to irritation ; the petiole had 

 less work to perform as the leaf was largely supported by 

 the object which its petiole had clasped. 



The effects of increased use may be observed in the 

 organs of special sense. When an individual loses an 

 eye in early life the remaining healthy eye acquires a 

 greater range of movement and quickness which com- 

 pensates in no small degree for the loss of its companion. 

 In persons blind from early life the power of hearing 

 becomes wonderfully quickened, and their tactile sensi- 

 bility is so heightened that they make themselves 

 acquainted with external surroundings in a marvellous 

 manner. Similar instances are furnished by the mole ; 

 its sense of hearing is proverbial. Says Caliban to 

 Stephano and Trinculo : " Pray you tread softly, that 

 the blind mole may not hear a footfall." The blind 

 fish of the mammoth cave are said to be abnormally 

 sensitive to sounds as well as to undulations produced 



