280 



SCIENCE. 



[N. S. Vol. XXIV. No. 609. 



hibited. In mice the relation of these pig- 

 ment characters is apparently reversed. If in 

 mice black and yellow are present together, 

 the activity of Uach is largely inhibited; 

 wholly so in the first coat and sometimes in 

 the second, though not in the eyes; partially 

 so in the adult pelage of sooty yellows. We 

 can not suppose that some factor of black, 

 wholly absent in the young, has been supplied 

 in the adult. If so, whence has it cOme ? Nor 

 can we suppose that any factor of black is 

 absent in either of two yellow mice, which 

 produce black offspring. Black was not ab- 

 sent, but inhibited! Removal of the inhibit- 

 ing character, yellow, allows black to resume 

 activity in the young. And yet we can not 

 suppose that yellow is in mice invariably in- 

 hibiting in its action on black. F6r yellow 

 pigment, as well as black, is formed in the coat 

 of wild gray mice. And Cuenot recognizes 

 that yellow may ' vary ' to such an extent as 

 to be very similar to the gray of wild mice. 

 Black is inhibited (dominated) only by yellow 

 in that peculiar state of activity which it has 

 in self-yellow mice, or, as I have elsewhere 

 (1905) expressed the matter, in that condition 

 of 'relative potency' which it has in yellow 

 mice. It is not impossible that a black race 

 of mice could be found (or created) in which 

 black had sufficient potency to dominate ordi- 

 nary yellow, as it does in guinea-pigs and 

 rabbits. Or, what would amount to the same 

 thing, a yellow strain might be formed so lack- 

 ing in relative potency (inhibiting effect) that 

 in crosses with it, black would remain fully 

 active. I have pointed out (1905) one similar 

 case (of partially reversed dominance) in 

 which certain smooth guinea-pigs form gam- 

 etes of such potency as to partially inhibit in 

 their cross-bred young the usually dominant 

 ' rough ' character of the coat. That the ac- 

 tion in question was a temporary one, one of 

 inhibition, was shown by the fact that the 

 rough character could be recovered in full in- 

 tensity in generation F^. 



Bateson, Saunders and Punnett have also 

 shown recently (1906) that white plumage in 

 poulti-y, ordinarily a dominant character, may 

 in certain strains function as a recessive. 



The different behavior of yellow, in relation 

 to other pigment types, in mice and in guinea- 

 pigs, becomes less surprising when we learn 

 that within the same species the relative po- 

 tency of a character may vary to such an ex- 

 tent that it functions sometimes as a domi- 

 nant, sometimes as a recessive character, in 

 crosses with the same alternative type. 



In several recent papers I have pointed out 

 the fact that the theoretical 'purity of gam- 

 etes ' of Mendelian inheritance does not exist. 

 No more does the purity of factors exist. We 

 can not avoid the idea of impurity of the gam- 

 etes, by introducing the conception of ' fac- 

 tors,' for the factors are as certainly impure 

 as the gametes. 



To sum the matter up, it is certain that 

 unit characters exist, but it is equally certain 

 that the units are capable of modification; 

 gametic segregation certainly occurs in some 

 cases (Mendelian inheritance), it does not 

 occur in others (blending inheritance) ; fac- 

 tors of characters certainly exist, when char- 

 acters are demonstrably complex' and result 

 from the coexistence of two or more simpler 

 ones, as, for example, a purple- pigmentation 

 due to coexistence of red' and of blue chloro- 

 plastids in plants. But let us in no case in- 

 troduce more factors into our hypotheses than 

 can be shown actually to exist. 



bibliogkaphy. 

 Allen, G. M. 



1904. The heredity of coat-color in mice. 

 Proc. Am. Acad. Arts and Sci., vol. 40, pp. 

 61-163. 



Bateson, W. 

 1903. The present state of knowledge of colour- 

 heredity in mice and rats. Proc. Zool. Soc., 

 London, 1903, 2, pp. 71-99. 

 Bateson, W., Saunders, E. R., and Punnett, 

 R C. 

 1906. Experimental studies in the physiology 

 of heredity. Reports to the Evolution Com- 

 mittee of the Royal Society, III., 53 pp., 4 figs. 

 Castle, W. E. 



1905. Heredity of coat-characters in guinea- 

 pigs and rabbits. Carnegie Institution Pub- 

 lications, No. 23, 78 pp., 6 pi. 



Cuenot, L. 



1905. Les races pures et leurs combinaisons 

 chez les souris. Arch^ de Zool. Exp. (4), torn. 



