22 Darbishire, Laws of Heredity. 



or Galtonian Law describes the facts of heredity depends 

 on the composition of the unit tested. There is nothing 

 a p nor 2 iWoQical in treating the sharply defined category 

 of dark-eye-and-coloured-coat, in mice, as a unit. Suppose 

 this is done : Galton's Law fits the facts beautifully, while 

 Mendel's is triumphantly refuted (by shewing that the 

 amount of albino ancestry of a hybrid affects the per- 

 centage of albinos produced by such hybrids mated 

 inter se) : to which the Mendelian would make the follow- 

 ing answer : " Are you sure that your unit ' dark-eye and 

 coloured-coat ' is incapable of resolution into still simpler 

 units. Are you sure that you are not regarding as a 

 simple thing that which is really compound, just as the 

 ' fixed alkalies ' were regarded as elements until Davy 

 showed them to be compounds.*^ I can prove that 

 you are ; for by testing the gametic constitution of the 

 dark-eyed and coloured-coated forms, I can shew that 

 they are sharply distinguished into heterozygous and 

 homozygous forms. Now I claim to have discovered 

 what should really be treated as a unit ; namely that 

 character which, like the elements of the chemist, 

 cannot be split up into simpler characters. 1 do 

 not pretend that my formulas of heredity describe the 

 numerical results obtained by jumbling a lot of my 

 elemental units together, any more than you pretend that 

 the Law of Ancestral Heredity describes the phenomena 

 exhibited by my units when dealt with separately." 



* Davy 'o8. 



+ This illustration may at first sight appear not to be strictly parallel. 

 A chemist reading it would think that what was going to be shewn was 

 that the unit "dark-eye-and-coloured-coat" was resolvable into "dark-eye" 

 and "coloured-coat," whereas, of course, what is really going to be shewn 

 is that the units "dark-eye and coloured-coat " are of two quite distinct 

 kinds. The true parallel to my case is the idea ol the element before 

 Davy's time. Amongst the things that were classed as elements, some 

 really were undecomposable (cf. the homozygous forms), while others — the 

 alkalies — were decomposable (cf. the heterozygous). 



