154 Experimental Zoology 



being exceedingly rare and almost confined to South Wales, but 

 that of T. crepuscularia is widely distributed and known as 

 aberratio delamerensis. The first generation of hybrids (F^) 

 is described as containing individuals that are like the one or 

 the other parent form, although each kind may show traces of 

 the other species. Tutt states that he has avoided the use of the 

 term "intermediate" when describing these hybrids because the 

 term has been made to cover so many different things ; but that 

 nevertheless all of the hybrids with few exceptions are inter- 

 mediate to varying degrees, i.e. " almost every specimen appeals, 

 in some part of its facies, to a specialist, as resembling T. bis- 

 tortata, whilst the same specimen, in other particulars, strikes 

 one as resembling T. crepuscularia." When inbred the hybrids 

 produce a large percentage of individuals differing much from 

 either parent-form. The crossing of the hybrids, obtained 

 from original reciprocal crosses, tends to produce a mixed prog- 

 eny, some referable to known forms of the crossed species, 

 others quite unlike anything ever obtained in nature. . . . These 

 experiments support Eimer's view that sexual combination can 

 lead to the production of new forms. I doubt, however, very 

 much whether they could be perpetrated without selection," 

 for if crossed with one of the present species they would "in my 

 opinion" revert to the wild form. 



Some interesting results were also obtained in regard to the 

 sex of the hybrids. "The greater vigor of the male results 

 largely in the production of female offspring. When the male 

 is of the dominant species, females are developed in fair propor- 

 tion ; when the female is of the dominant species, males are 

 largely in excess." 



LITERATURE, CHAPTER IX 



COUTAGNE, G. Recherches experimentales sur 1'here'dite' chez les vers 

 ^ soie. Bull. Sci. France et Belg. XXXVII. 1902. 



Sur les facteurs e'lementires de 1'heredite. Comp. Rend. Acad. Sci. 

 Paris, CXXXVII. 1903. 



Sur les croisements entre taxies differentes. Comp. Rend. Acad. Sci. 

 Paris, CXXXVII. 1903. 



