80 GENETICS 



chicks out of "wild" eggs, asks the pertinent ques- 

 tion: "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 difficult to see how cau- 

 tion, gained by the experience of the parents, can find 

 its way into the fertilized egg. If, then, some instincts 

 require a different explanation from that of "inherited 

 habit," may it not be likely that all instincts do? Is 

 it not better to assume that the structure of the germ- 

 plasm determines a particular response to a particular 

 stimulus regardless of whether in the past the ances- 

 tors have made a similar response to a similar stimulus ? 



d. Transmission of Disease 



If acquired diseases were heritable we would all have 

 been dead long ago. When a son, whose father died 

 of pneumonia, succumbs himself to pneumonia after 

 an interval of years there may be no more causal 

 or hereditary connection between the two events than 

 when a second house burns down on the same site where 

 a former house went up in flames. 



Many diseases, like tuberculosis, have their imme- 

 diate cause in invading pathogenic bacteria. Bacteria 

 themselves cannot be inherited for the reason that it 

 is not possible for them to become an integral part of 

 the fertilized egg and thus cross the "hereditary 

 bridge" which joins two generations. A general pre- 

 disposition to bacterial disease, that is, a lack of re- 

 sistance to bacterial invasion due to defectiveness in 



