40 i 



SCIENCE. 



[N. S. Vol. X\ail. No. 456. 



Ratios of Dominants to Eecesdves.— Con- 

 siderable departures are to be expected 

 when the number of offspring taken into 

 consideration is small, but with increase in 

 the number of offspring examined, the de- 

 partures should grow less. This is usually 

 found to be true. Mendel's numbers are 

 shown by Weldon ( :02) to be well within 

 the limits of probable error. But certain 

 cases have been observed in which depart- 

 ures of a particular sort persist even with 

 considerable numbers of offspring. Thus 

 Allen and I have found the recessive char- 

 acter, white, in mice to be inherited in about 

 three per cent, more than the calculated 

 number of eases, while the equally reces- 

 sive dancing character is inherited in about 

 thirty-three per cent, less than the calcu- 

 lated number of cases. These fairly uni- 

 form departures indicate, to my mind, a 

 vitality, on the part of the recessive 

 gamete, in one ease somewhat superior, in 

 the other much inferior, to that of the 

 dominant 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 ex- 

 tended investigations 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 re- 

 cessives, instead of hybrids. Such cases 

 are, I believe, correctly referred by Bate- 

 son to the category of 'false hybridiza- 

 tion' as described by Millardet, a phe- 

 nomenon akin to parthenogenesis, in which 

 sexual union has served merely to stimu- 

 late 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 reces- 

 sive. Tschermak ( :01) believes that he 

 has found a few such cases among cross- 

 bred beans. Sex and certain other di- 

 morphic 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 importance 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 disintegration on the part of 

 any or all of the characters of the species, 

 and the disintegrated characters may, in- 

 deed 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 char- 

 acters 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 or- 

 ganisms 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 in- 

 herit some characters dominant, others 

 latent, and polymorphism of the race re- 

 sults. Only selection continued through 

 long periods of time will serve to eliminate 

 completely the latent recessives, and so to 

 cause the disappearance of certain aber- 

 rant variations. 



Bateson makes the pregnant suggestion 

 that even cases of continuous variation 

 may possibly prove conformable Vvith Men- 



