MENDELIAN HEREDITY 271 



non-sexual characters came to be inherited by the 

 above-outlined sexual mode of inheritance. So long 

 as the only character which existed in the DR con- 

 dition was one of the sexes, the DE condition could 

 never unite with anything but the RR condition, 

 which was that in which the other sex existed. If 

 that of the $ was the bipotential one, only one kind 

 of mating could take place, DR ( $) x RR ( ^j). But 

 directly a non-sexual character acquired the bipoten- 

 tial condition the fat was in the fire. It became 

 possible for the DR condition to mate with the DR 

 condition, and this led to the origin of the pure 

 dominant and recessive types, DD and RR, which, 

 as being perfectly constant, would be greedily 

 seized on by the breeder so soon as such types came 

 under his intelligent observation. And the existence 

 of these two pure types, DD and RR, suggested them- 

 selves as suitable material for hybridisation experi- 

 ments, probably hundreds of years after they were 

 isolated, to Mendel, who tabulated the results of his 

 crosses and discovered the phenomenon (illustrated 

 on the frontispiece of this book) which we are apt 

 to regard as so fundamental. Mendelian segregation 

 in the ratio 1:2:1 only became possible when it 

 became possible for a DR to mate with a DR ; and 

 this only became possible when the DR condition 

 ceased to be the exclusive attribute of, say, femaleness, 

 and became an attribute of, say, yellowness. Men- 

 delian segregation in the ratio 1:2:1 could never 

 take place before this event, because a DR could not 

 mate with a DR, both being females ; but once it 



