INHERITANCE 229 



discovered already, stated with the utmost clearness and 

 precision by the Augustinian monk, Gregor Mendel, 1 as 

 early as 1865, though it had been completely forgotten 

 ever since. 



The so-called " rule of Mendel ): is based upon experi- 

 ments with hybrids, that is, with the offspring of parents 

 belonging to different species, or, at least, varieties, but it 

 relates not to the characters of the generation resulting 



O o 



immediately from hybridisation, the "first" generation of 

 hybrids, as we shall call it, but to the characters of that 

 generation which is the result of crossing the hybrids with 

 each other, provided that this leads to any offspring at all. 

 There are many cases indeed, both amongst animals and 

 plants, where the offspring of the hybrids, or in other 

 terms the " second " generation, is found to consist of 

 individuals of three different types the mixed 2 type of 

 the hybrids themselves, and the two pure types of the 

 grandparents. Whenever the individuals of the "second" 

 generation are separated into these three different types, 

 hybrids are said to " split." It is the fact of this splitting 

 on the one hand, and on the other hand a certain statement 

 about the numbers of individuals in the three different 

 types of the " second " generation, that gives its real 

 importance to Mendel's rule. 



Before discussing what may follow from Mendel's 

 discovery for the theory of heredity, we must lay stress 



1 New edition in the " Klassiker d. exakt. Wiss." Leipzig, Engelmann ; 

 see also Bateson, Mendel's Principles of Heredity, Cambridge, 1902. 



2 For the sake of simplicity I shall not deal here with those cases of 

 hybridisation in which one quality is "recessive," the other "dominant," 

 but only allude to the cases, less numerous though they be, where a real 

 mixture of maternal and paternal qualities occurs. 



