No. 593] SHORTER ARTICLES AND DISCUSSION 



311 



color patterns. Like produces like, but under natural conditions 

 only by skipping a generation. Except for the innate tendency 

 for the types to alternate, the case is similar to that of the red 

 primrose described by Baur 2 which, growing at 15°-20° C. pro- 

 duces red flowers, at 30°-35° C, white. Or it is like the mutant 

 stock of Drosophila described by Miss Hoge, 3 which, bred in win- 

 ter or in an ice chest, gives a large proportion of flies with super- 

 numerary legs, though in summer or in moderate temperature 

 the stock appears to be normal. The same set of factors under 

 varying conditions produces different results. An analysis of 

 the factors underlying another similar case in Drosophila, "ab- 

 normal abdomen," has been worked out by Morgan. 4 This re- 

 markable mutant was shown to behave as a dominant sex-linked 

 character. It manifests itself, however, only when the food in 

 which the flies are bred is kept moist. 



The rhythmic tendency of prorsa to produce levana, notwith- 

 standing artificial raising of the temperature, shows that this 

 is a sort of alternation of generations in which the definitive sex- 

 ual generation, prorsa, alternates with another apparently more 

 primitive, levana, which is also sexual. This seasonal alternation 

 of sexual forms in its hereditary basis is comparable to typical 

 alternation of asexual and sexual types. 



Weismann, 5 in discussing the case cited, assumed the presence 

 simultaneously in the germ plasm of prorsa-determinants and 

 Zeucma-determinants. 



But these prorsa-ids were at the same time so arranged that they be- 

 came active under the action of a higher temperature, if this is acting 

 at the beginning of the pupal period, while the levana-ids become active 

 at a lower temperature. Heat, therefore, is only the excitant which 

 sets free the prorsa-determinants, while cold sets free the levana-de- 



Modernizing Weismann 's hypothesis, may we suppose that 

 distinct Mendelian factors underlie each of these two discontinu- 

 ous types of coloration? The idea is attractive, but all the evi- 

 dence at hand indicates that the determinants or factors of both 

 types are borne by all the gametes. Intermediates, showing a 



2 .' ' Einf iihrung in die Vererbungslehre, ' ' pp. 4-6. 



s Jour. Exper. Zool., 18, 1915. 



*" Mechanism of Mendelian Heredity," pp. 39-41. 

 5 "New Experiments on the Seasonal Dimorphism of Lepidoptera. " 

 Translation by Nicholson in the Entomologist, 1896. 



