CAUSES OF GENETIC VARIATION 221 



lation. The variations in these cases were evidently fluctu- 

 ational. In some instances, however, real genetic differences 

 were met with, and strains exhibiting them were, as usual, 

 rapidly fixed. 



Tower points out that several of the varieties (or species, as 

 he prefers to call them) were obviously recessive to decemlineata. 

 This is most clearly demonstrated in the case of the form called 

 pallida, which is a pale depauperated-looking creature, with the 

 orange of the thorax almost white and the eyes devoid of pig- 

 ment. 9 This form behaved as an ordinary Mendelian recessive, 

 breeding true whenever it appeared in the cultures, or when 

 individuals found wild were studied in captivity. A black form 

 which Tower names melanicum was similarly shown to be a 

 Mendelian recessive. Wild specimens of this variety of opposite 

 sexes were not found simultaneously in nature, and there was thus 

 no opportunity of breeding them together, but the hereditary 

 behaviour was seen in the F 2 generation from a melanicum found 

 coupled with decemlineata. Experiments also occurred giving 

 indication that a variety with the stripes anastomosing in pairs 

 (tortuosa), was another recessive, and that a variety — called 

 " rubri-vittata" — gave an intermediate Fi with subsequent 

 segregation. All these are forms of decemlineata Stal. 



Similar observations were made regarding forms recessive to 

 multitaeniata Stal. Of these two were thrown by multitaeniata 

 itself, namely a form named by Stal melanothorax, and regarded 

 by him as a species, and one which Tower names rubicunda n. sp. 

 The facts proving the recessive behaviour of their several forms 

 will be found in the following places in Tower's book: 



pallida, pp. 273-278. 



melanicum, p. 279. 



tortuosa, p. 280. 



rubrivittata, pp. 280-281. 



melanothorax and rubicunda, pp. 283-285. 



Following this evidence of recessive nature of the six forms 



9 This is indicated in the coloured plate, but I have not found any explicit state- 

 ment to this effect in the text, and am not sure if the absence of pigment was re- 

 garded as complete. 



