The progress of Geuetics since the rediscovery of Mendel's papers. 387 



It will be imderstood that the main ])oint, the existence of distinct 

 kinds of albinos bearing distinct factors is now perfectly clear. 



The researches ^) of C u é n o t and Hurst, together with the 

 elaborate experiments of Miss F. M. Durham^) as yet unpublished, 

 have fully demonstrated this fact in Mammals, and as we shall see, 

 the same is true of several plants. We have now the means of 

 proving- the composition of each albino by appropriate breeding' tests. 

 For instance, when bred to a pure black, a G albino will give greys 

 only; a GB albino will give equal numbers of greys and blacks, while 

 a B albino will give blacks only. Conversely it is also possible to 

 manufacture by suitable matings, albinos of each composition. For 

 example, albinos extracted from chocolates can only bear the choco- 

 late determiner. Those from black mice must all bear the black 

 determiner if the families have been large and no chocolates have 

 occured. Those from grey mice must all bear the grey determiner, 

 if in sufficient numbers no blacks or chocolates have been produced. 



It is this fact, that in most cases the albino will be bearing the 

 determiner proper to the colour of the last coloured parent from 

 which it was extracted, that has led Castle (18) and others to 

 speak of these colours as "latent" in the albino. This mode of ex- 

 pression is much to be regretted and not only introduces confusion 

 into the discussion of the problem but is positively misleading.-^) 

 There is no "latency" e. g., of black or grey, as a whole, in the 

 albino. The albino bears one of the factors necessary to the pro- 

 duction of black or of grey. Sulphate of copper is blue and chloride 

 of copper is green, but it would be improper to speak of blue as 

 latent in sulphuric acid, or of green as latent in hydrochloric acid. 



^) Keference should here he made to the vast researches lately published by 

 Haacke (58). Before the rediscovery of Mendel his experiments on mice had led 

 him to correct conclusions on all the essential points, and it is . much to be regretted 

 that the publication of this remarkable work was so long- delayed. 



^) Among other interesting points, Miss Durham has investigated the inheri- 

 tance of the various degrees of concentration of pigment, to which many of the 

 peculiar colours of mice are due. For example black may exist in the concentrated 

 state called „black", or in the dilute condition called „blue". Similarly „Chocolate" 

 is the concentrated state of the pigment which when dilute is called „Silver-fawn". 

 Concentration depends on a definite factor dominant to dilution. Thus (1) black X 

 silver-fawn, and (2) blue X chocolate both give Fi black; and both give F^ consisting 

 of 9 black: 3 chocolate: 3 blue: 1 silver-fawn. „Concentration" or „dilution" may 

 similarly be introduced by means of an albino; e. g.. silver-fawn X albino bearing- 

 concentration gives black Fi, and in F.^ these give the ratio 27 black: 9 chocolate: 

 9 blue : 3 silver-fawn : 16 albino. 



•'') Morgan (79) has evidently been misled by this use of the term latency 

 when be writes. „The most important fact that cannot be interpreted on the 

 assumption of pure germ-cells is found in the heredity of extracted récessives." 

 Biol. Cbltt., XXVI, 1906, p. 289. 



