INHERITANCE OF ACQUIRED CHARACTERS 91 



in environment simply permits the life-cycle to go 

 on further. Changing from gill-breathing to lung- 

 breathing is not, therefore, an acquired character, 

 but a purely germinal character that may be either 

 blocked or released by changing conditions in the 

 environment. 



c. The Effects of Use or Disuse 



The callosities on the end of a violinist's left-hand 

 fingers are acquired by use, but they are not inherited. 

 There are callosities on the knees of the wart-hog, 

 Phacechoerus, which are also apparently the result 

 of use, for these animals kneel as they root for a living 

 in the African forests, and have done so for untold 

 generations. It has been noticed that young wart- 

 hogs as soon as they are born possess the callosities, 

 so that this instance looks like one of inheritance of 

 a character acquired through use or exercise. 



The skin on the soles of human feet is thicker than 

 the skin elsewhere, and by use it becomes still thicker. 

 This is apparently another instance of the same sort. 

 The writer has observed, however, that a cross sec- 

 tion through the foot of a "mud puppy," Necturus 

 maculatus, shows a much thickened sole. Necturus, 

 it should be noted, is a very primitive salamander 

 living always under water and never using the soles 

 of its feet in any way to bear its weight, nor is it 

 reasonable to suppose that it ever had any ancestors 

 who did so, for the hands and feet of the Amphibia 

 are the most primitive and ancient hands and feet to 

 be found in the animal kingdom without any known 



