130 Heredity and Eugenics 



seemingly very constant and uniform. Perhaps the most 

 famous of these are the blackberry-raspberry 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 (Rubus}, 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 Mendel or not. The 

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

 point, but several hypothetical explanations of the phe- 

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

 the universality of Mendelianism. 



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 nth 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 hybrids 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 

 hypothetical case, suppose two plants are crossed in which 

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



