206 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of guinea-pigs have originated by loss variations or loss mutations. It 

 does not follow, however, that the wild species have originated in this 

 way. The two wild species, G. cutleri and C. rufescens, are probably 

 distinct enough to show interspecific sterility, since one is known to 

 form sterile hybrids, the other fertile hybrids, in crosses with the 

 guinea-pig. Their specific distinctness accordingly cannot be due to 

 such mendelizing factors as distinguish one domesticated variety from 

 another, but to something more fundamental in character, though less 

 striking in appearance. 



Castle also reports oh hybridization experiments with a race of feral 

 guinea-pigs from lea, Peru, and with a domesticated Arequipa race, and 

 one of his general results is to confirm his previous conclusion (1909) 

 that size inheritance is blending and does not mendelize. This does 

 not preclude the possibility that in special cases mendelizing factors 

 may exist which affect size. According to Castle, the hypothesis of 

 quantitative variations in a blending character presents fewer difficulties 

 as an explanation of size inheritance than the hypothesis of multiple 

 unvarying segregating factors. His observations on rats and other 

 rodents (1915) may be cited to show that even single mendelian unit 

 characters are quantitatively variable. 



The second study is by Sewall Wright, and deals with the inheritance 

 of colour and of other coat characters in guinea-pigs, with especial 

 reference to graded variations. He deals with the factors determining 

 coloration, and with the various ways in which intermediates may arise 

 between varieties which mendelize regularly. A complex of the most 

 varied causes may underlie an apparently simple continuous series of 

 variations. 



In another set of studies. Castle deals with piebald rats and selection 

 and with gametic coupHng. Experiments with hooded rats point to the 

 conclusion that the factor for hooded colour pattern may fluctuate in 

 genetic value. Hence racial changes may be effected through selection 

 by the isolation of genetic fluctuations, as well as by the isolation of 

 mutations. Moreover, genetic fluctuation makes possible progressive 

 change in a particular direction, repeated selection attaining results 

 which it would be hopeless to seek by any other means. There is no 

 natural limit to the progress which selection can make in changing the 

 hooded character. It was found, however, that the long-continued 

 inbreeding of the selected races affected their vigour and fecundity, as 

 was observed by Darwin, Bos, Weismann, and others. In conclusion 

 Castle discusses two yellow mutations in rats which show "mutual 

 repulsion " in heredity. 



Early Larval Stages of Eels.* — Johs. Schmidt has been able to 

 throw further light on the life-history of the fresh-water eel, both the 

 European eel (Anguila vulgaris) and the American eel {A. rostraia). 

 In the case of the former, especially, his material is very abundant, 

 embracing all stages, from a length of scarcely 9 mm. upwards to full- 

 grown larvae 88 mm. long. The succession is so complete that every 



* Medd. Komm. Havund., Bd. v. (1916) pp. 1-20 (4 pis.). 



