An Introduction to a Biology 



Before continuing my argument, I should like to dwell 

 for a little on the circumstances which led me to believe 

 that I was dealing in F2 with hybrids, and, indeed, with 

 heterozygotes in the strict Mendelian sense of the term. 

 In the first place, these putative hybrids bore the same 

 features of coat and eye colour as the F^ hybrids exhibited ; 

 secondly, they formed 50% of the F^ generation ; and 

 lastly, at least two upholders of Mendelian theory ^ had 

 asserted that the heterozygote was represented in my ex- 

 periment by the coloured mice with pigmented eyes. 



To resume the thread : these results were brought before 

 the notice of a student of heredity whose first question was, 

 Had there ever resulted from the miion of two hybrids a 

 family in which there were no albinos 1 — a query to which 

 an affirmative answer was given. Then the following argu- 

 ment was used by my critic : " The only proof that a given 

 individual is a hybrid is one which is based on an examina- 

 tion of its gametic constitution ; in the case of your mice 

 you have no right to say that a grey mouse with black eyes 

 is a hybrid until you have mated it with an albino and obtained 

 albino yoimg in the litter thus produced ; until this has 

 been done there is no evidence that it is not a dominant. 

 In the case you have just shown me 3^ou mated coloured 

 mice with dark eyes without making this test, and by this 

 neglect many dominants may have been included among 

 them, and you see that this suggestion, against the truth 

 of which you have no evidence, accounts just as well as 

 Galtonian theory for the difference in the proportions of 

 albinos in the three kinds of matings." I repHed that the 

 test of the true heterozygote nature of the apparent hybrids 

 should be made, though I did not believe the suggested 

 Mendelian interpretation of this apparently conclusive anti- 

 Mendelian result. 



The test has been applied with the most remarkable 

 result ; but before giving an account of it I propose to describe 



1 Castle and Allen, " Proc. Amer. Acad. Arts and Sci.," Vol. 38, No. 21. 



149 



