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- 

 dclians 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 1 DD (pure or extracted dominants) -f- 2 D(R) 

 (impure dominants) -f- 1 RR (pure or extracted recessives). 



