92 GENETICS 



ancestral types. The thickening of the skin on the 

 sole of the mud puppy's feet must be due, therefore, 

 to germinal determiners and is in no way an acquisi- 

 tion through use. The same may also be true of the 

 wart-hog's knees and of human soles. 



The strong arm, the skilled hand, and the trained 

 ear are not inherited. They have always to be 

 reacquired in each succeeding generation just as 

 surely as the ability to walk, or to read and write. 



Herbert Spencer has defined instinct as "inherited 

 habit." But surely those instincts which determine 

 a single isolated action during the lifetime of the 

 individual, such as the spinning of a peculiar cocoon, 

 cannot be the result of habit, since habits are formed 

 only through repeated action. If, then, some in- 

 stincts require a different explanation from that of 

 "inherited habit," may it not be likely that all in- 

 stincts do ? Dr. Hodge, who succeeded in hatching 

 tame quail chicks out of "wild" eggs, asks the perti- 

 nent question: "How can a fear hatch out of an 

 egg?" The habit of wildness, particularly with 

 precocial chicks like quails, may, under an inciting 

 environment, be very soon established, but it is diffi- 

 cult to see how caution, gained by the experience of 

 the parents, can find its way into the fertilized egg. 



d. Disease Transmission 



Many diseases, like tuberculosis, have their im- 

 mediate cause in invading pathogenic bacteria. 

 Bacteria themselves cannot be inherited for the 

 reason that it is not possible for them to become an 



