No. 546] INCONSTANCY OF UNIT-CHj 



equally in each case, upon crossing the two quantitatively 

 different stages. It is the substantial integrity of a 

 quantitative variation from cell-generation to cell-gen- 

 eration that constitutes the basis of Mendelism. All else 

 is imaginary. 



We can distinguish and trace the history of these 

 quantitative variations from generation to generation 

 only when the differences between them are of some size. 

 This has led many to think that only variations of some 

 size are inherited (the mutation theory) and others to 

 deny that such variations can be increased in size by se- 

 lection (the genotype theory). < >thers still observing un- 

 mistakable evidence that small variations are heritable 

 no less than large ones conceive that the large variations 

 which can be increased or decreased by selection are com- 

 posed of a certain number of smaller ones cumulative in 

 their effects (the multiple factor hypothesis). A fatal 

 objection to this idea is the fact that these quantitative 

 variations behave as simple units, not as multiple ones, 

 and so give mono-hybrid ratios, not polyhybrid ones. 

 The only logical escape from this dilemma for one de- 

 voted beyond recall to a pure-line hypothesis will be to 

 assume further that the assumed multiple units are all 

 coupled, i. e., all united in a single material body so that 

 in cell-division they behave as one unit, for practical 

 purposes are one unit. This position will be logically 

 unassailable, for we shall never know whether the bodv 

 which in practise behaves as one is in the last analysis 

 composite. Chemists tell us (or used to) that water is 

 composed ultimately of atoms of hydrogen and atoms of 



weight and hard and indestructible as rocks; neverthe- 

 less for practical purposes we drink our cup of water and 

 do not chew it. I for one will be content with the ad- 

 mission that variation is as continuous as water and will 

 not press the argument against discontinuity into realms 

 of the ultimate. 



The majority of the characters dealt with by the ani- 

 mal breeder are less simple in behavior than color char- 



