SUSCEPTIBILITY AND IMMUNITY 123 



to the lack of a character mated with a like indi- 

 vidual will produce offspring all of which lack the 

 character. 



Wherefore it follows that if an individual pos- 

 sessed of an hereditable character (in double dose) 

 mate with another which lacks that character, 

 the offspring will all exhibit that character, but if 

 these offspring mate among themselves their imme- 

 diate descendants will comprise some which have the 

 character and some which lack it. If the character 

 be an objectionable one, susceptibility to a certain 

 disease, for example, the mating of a susceptible, 

 with an immune individual will produce what by 

 every rule of common sense would be called a 

 disastrous result. The offspring will be susceptible 

 and common sense if it had no other guidance would 

 seek elsewhere for the means of eradicating suscep- 

 tibility to disease from a strain of plants. But 

 seen in the light of Mendelian knowledge the imme- 

 diate failure of the experiment is a promise of 

 subsequent success. The fact that all the plants 

 in the first generation suffer from the disease is 

 evidence that the character, susceptibility to 

 disease is Mendelian in its behaviour and a sure 

 guarantee that if individuals of the first generation 

 are bred together in pairs their offspring will com- 

 prise some which possess and exhibit the immunity 

 of the grandparent. Need it be said that it was by 

 applying this knowledge of the Mendelian laws of 

 inheritance and of the nature of plant disease that 



