354 EVOLUTION, GENETICS, AND EUGENICS 



One variety, Zea mays alba, which has smooth white seeds, was 

 crossed with another variety, Zea mays coeruleodulcis, which has 

 wrinkled blue seeds. The hybrids (Fi) had smooth blue seeds, one 

 character of each parent being dominant, and one character of each 

 parent being recessive. The hybrids were inbred, and the progeny 

 (Fa) showed four combinations — smooth blue, smooth white, wrinkled 

 blue, and wrinkled white (the dominant characters are itaUcised). 



In the next generation (Fj), the wrinkled white, inbred, yielded 

 wrinkled white — a case of extracted recessives, breeding true. The 

 smooth whites and wrinkled blues, inbred, yielded partly forms like 

 themselves and partly wrinkled white. The smooth blues, inbred, 

 yielded the same combinations as in Fj. 



A finer corroboration of Mendelian could hardly be wished. 



Nettles. — Correns crossed two "species of stinging-nettle," Urtica 

 pilulijera L. and U. dodartii L., which resemble one another except as 

 regards leaf-margin, strongly dentate in the former, almost entire in 

 the latter. The hybrid offspring (Fi) have all dentate leaves like the 

 male or the female parent, as the case may be. The dentate character 

 is absolutely dominant. The inbred (self-fertilised) hybrids produce 

 offspring (FJ of two kinds, with dentate and with entire margins, on an 

 average in the Mendelian proportion, 3 : i. 



"Immunity to rust in wheat. — Some kinds of wheat are very 

 susceptible to the fungoid disease known as ' rust ' ; others are immune. 

 The quality of immunity to rust is recessive to the quality of predis- 

 position to rust. 



" When an immune and a non-iiimiune strain are crossed together 

 the resulting hybrids are all susceptible to * rust.' On self-fertilisation 

 such hybrids produce seed from which appear dominant 'rusts' and 

 recessive immune plants in the expected ratio of 3:1. From this 

 simple experiment the phrase 'resistance to disease' has acquired a 

 more precise significance, and the wide field of research here opened 

 up in this connection promises results of the utmost practical as well 

 as theoretical importance. To the question, ' Who can bring a clean 

 thing out of an unclean ? ' we are beginning to find an answer, nor is 

 the answer the same as that once given by Job" (R. C. Punnett). 



Silkworms. — Toyama paired Siamese silkmoths with yellow or 

 with white cocoons; the offspring produced only yellow cocoons. 

 When the hybrids were inbred, the result was two sets, one producing 

 white cocoons, the other producing yellow cocoons, and the proportion 

 was Mendehan — 25.037 white and 74.96 yellow. The whites bred 



