1 3° Heredity and Eii italics 



seemingly ver}- constant and iniiform. Perhaps the most 

 famous of these are the blackberr}'-ras})berry hybrids first 

 produced by the late E. S. Carman and later by Luther 

 Burbank and others. Several hybrids having a commer- 

 cial value have been made in this genus {Riibiis), and the 

 small number of second generation progeny that have been 

 grown are said to have bred approximately true. Prac- 

 tically, it makes little difference about the exactness of this 

 statement. One can simply say that for all ordinary intents 

 and purposes, such hybrids breed true. To the scientist 

 it makes a great deal of difference whether these hybrids 

 are definite exceptions to the law of ^lendel or not. The 

 few data that we have are not sufficient to clear up this 

 point, but several hy]3othetical explanations of the phe- 

 nomena can be given that are in harmony with a belief in 

 the universality of JMendelianism. 



Nothing is really known about segregation in these 

 hybrids because the variations that occur are difficult to 

 describe and because the plants have never been grown in 

 large quantities. It is likely that numerous separately 

 heritable characters are concerned in such crosses between 

 true species, and when n pairs of character are concerned 

 it takes four to the /;th power seedlings to run an even 

 chance that there will be one plant like each of the parents. 

 When one considers that with ten pairs of characters, this 

 means over 1,000,000 individuals, he can see what enormous 

 numbers are needed to give valid conclusions. Moreover, 

 these h}'brids are only partially fertile and some considera- 

 tion must be given the possibility that selective fertilization 

 among the gametes of the hybrid may occur. To take a 

 h)'pothetical case, suppose two plants are crossed in which 

 the flowers of one are twice the length of the flowers of the 



