348 EXPERIMENTAL STUDY OF INHERITANCE 



recessive gametes." A black guinea-pig is dominant over an 

 albino guinea-pig, all the offspring being black ; it is probable 

 that this should be expressed by saying (with Castle) that " a 

 distinctive something of the black parent dominates a corre- 

 sponding nothing of the white parent." But it is especially 

 when we pass beyond such simple cases that the advantages 

 of the presence-and-absence conception over the original Men- 

 delian contrast are seen. 



It will, of course, be clearly understood that the facts of 

 Mendelian inheritance remain secure, though the interpretation 

 of what is meant by dominance or of segregation itself may 

 have to undergo modification. Thus we may refer to Dr. 

 Berry Hart's independent interpretation (1909) (which Men- 

 delians will not accept) of admitted Mendelian phenomena. 



Mendel's Theory summarised. Mendel discovered an im- 

 portant set of facts, and he also suggested a theoretical inter- 

 pretation the theory of gametic segregation. As Mr. Bateson 

 says, " The essential part of the discovery is the evidence that 

 the germ-cells or gametes produced by cross-bred organisms 

 may in respect of given characters be of the pure parental types, 

 and consequently incapable of transmitting the opposite char- 

 acter ; that when such pure similar gametes of opposite sexes 

 are united in fertilisation, the individuals so formed and their 

 posterity are free from all taint of the cross ; that there may 

 be, in short, perfect or almost perfect discontinuity between 

 these germs in respect of one of each pair of opposite characters." 



3. Corroborations 



Impure Dominants bred with Pure Types. In the typical 

 cases discussed above, a hybrid form D(R) an impure dominant 

 is supposed to be self-fertilised or inbred. The results are accord- 

 ing to the formula I DD (pure or extracted dominants) + 2 D(R) 

 (impure dominants) + i RR (pure or extracted recessives). 



