596 Scientific Proceedings, Royal Dublin Society. 



blacks and yellows in equal numbers, the latter blacks, yellows, and tortoises 



in the ratio 2:1 : 1. The experimental results are' : — 



Numbers of Parents. Progeny produced. 

 , ^"^ . 



Black. Yellow. Tortoise. 



3 5 8 — 



4 10 8 7 



The experiments are again in close accordance with the expected results. 



These same two black groups were also mated with tortoise and orange 

 (whose constitution as regards this experiment is ^x>jy). From this mating 

 only yellows and tortoises should result. It liappened so in the experiments.' 



The figures for each kind are not given. 



X' X 

 Thus our hypothesis that the constitution of JSTo. 28 is y y ^^^ ^^en 



tested six times — three by agouti-black matings and three by matings 

 with what have been called agouti-bearing blacks ; and, since at every test the 

 hypothetical and the experimental results have been in accordance, it may 

 reasonably be claimed that the hypothesis is sound. It follows, of course, 



X.'X 

 that the constitution of the original agouti-blacks is -p^ • If further 



experiments were thought necessary, the following might be suggested : — 

 (n) If , were mated with pure agouti, „ „, the progeny should 



all be agouti-blacks, i.e. y . 



(b) If Y Y ^®^® mated with pure black, , the progeny again should 



all be agouti-blacks. 



(c) If, however, these two blacks homozygous for JTand y on the one hand, 

 and for X' and F on the other, have not yet been found, the agouti-black 

 which is homozygous for Y {X'X YY) and has been found' should be mated 

 with itself. The progeny should be 1 black : 2 agouti-black : 1 agouti,, 



vi X' X' X X X 



that is, 1 Y^Y ' '^ Y Y ' ^ Y Y' '^^^ constitutions of the blacks and 



agouti-blacks could be tested readily. 



The suggestion may now be permitted that it is next to impossible to 

 identify the functions of the factors operating in this case. The functions of 

 the two recessives cannot be named. When they concur, tortoise is produced, 

 but other factors may be operating at the same time. Even if not, the effects 

 of X and y could not be told, since tliey cannot be separated. Nor can much 



1 Journal of Genetics, vol. ii, p. 233. 2 Jbid. p. 234. 



^ That is the first group in table ix, Journal of Genetics, vol. ii, p. 233. 



