castle. — mendel's law op heredity. 545 



gamete. Inferior vitality of gametes of either sort would result in greater 

 mortality and so in a diminished number of individuals derived from such 

 gametes. 



Of course other explanations are possible, as, that the two sorts of 

 gametes are not produced in equal numbers. More extended investiga- 

 tions of such cases can alone make their meaning clear. 



6. Reversal of Dominance. Exceptional cases are on record in which 

 crossing of a dominant with a recessive has resulted in the production of 

 pure dominants, or recessives, instead of hybrids. Such cases are, I 

 believe, correctly referred by Bateson to the category of " false hybridi- 

 zation " as described by Millardet, a phenomenon akin to parthenogenesis, 

 in which sexual union has served merely to stimulate one gamete to 

 development without bringing about its union with the other gamete. 



It is possible, however, that there are cases in which one of a pair 

 of characters is sometimes dominant, sometimes recessive. Tschermak 

 (:0l) believes that he has found a few such cases among cross-bred 

 beans. Sex and certain other dimorphic conditions found in the higher 

 animals and plants may prove to be cases of this sort. 



Acceptance of Mendel's principles of heredity as correct must lead 

 one to regard discontinuous (or sport) variation as of the highest im- 

 portance in bringing about polymorphism of species and ultimately of 

 the formation of new species. 



A sport having once arisen affecting some one character of a species, 

 may by crossing with the parent form be the cause of no end of disinte- 

 gration on the part of any or all of the characters of the species, and the 

 disintegrated characters may, indeed must, form a great variety of new 

 combinations of characters, some of which will prove stable and self- 

 perpetuating. Even if a particular combination of characters is uniformly 

 eliminated by natural selection under one set of conditions, it may 

 reappear again and again, and finally meet with conditions which insure 

 its success. 



We now have an explanation of the long-recognized principle that 

 new types of organisms are extremely variable, whereas old types vary 

 little. A new type which has arisen as a sport will cross with the parent 

 form. The offspring will then inherit some characters dominant, others 

 latent, and polymorphism of the race results. Only selection continued 

 through long periods of time will serve to eliminate completely the 

 latent recessives, and so to cause the disappearance of certain aberrant 

 variations. 



Bateson makes the pregnant suggestion that even cases of continuous 

 vol. xxxvni. — 35 



