142 THEORY OF EVOLUTION 



carried a red bearing X (black X in the fig- 

 ures) it will be seen that there should result an 

 exceptional class of sons that are red, and 

 an exceptional class of daughters that are 

 white. Tests of these exceptions show that 

 they behave subsequently in heredity as their 

 composition requires. Other tests may also 

 be made of the other classes of offspring. 

 Bridges has shown that they fulfill all the re- 

 quirements predicted. Thus a result that 

 seemed in contradiction with the chromosome 

 hypothesis has turned out to give a brilliant 

 confirmation of that theory both genetically 

 and cytologically. 



How Many Genetic Factors are there in 

 THE Germ-plasm of a Single Individual 



In passing I invite your attention to a 

 speculation based on our maps of the chromo- 

 somes — a speculation which I must insist does 

 not pretend to be more than a guess but has at 

 least the interest of being the first guess that 

 we have ever been in position to make as to 

 how many factors go towards the makeup of 

 the germ plasm. 



