1 lo PlantjSenetics 



develop two diverse strains by selection, for selection 

 would result in piling up the cumulative factors in one 

 direction or another. Castle's rejoinder would be that 

 if this is a cumulative factor situation why do none of 

 the extremes appear in the non-selected stock, which 

 instead breeds approximately true within very narrow 

 limits of variabiUty? The answer is made that the 

 extremes do not appear in the pure-bred stock merely 

 because of mathematical possibiUty. If we are dealing 

 with six cumulative factors, and the so-called pure 

 stock has an intermediate number of doses, there 

 could not be much chance of getting out the extremes 

 in later generations. It will be remembered that it 

 would be necessary to secure over 4000 progeny to 

 have an even chance of getting one such extreme; or 

 at least 50 progeny to get anything that would visi- 

 bly approach the extreme. It would seem, therefore, 

 that Castle's chances to determine this would be very 

 small. Rats certainly do not produce 4000 progeny 

 in a single generation; in fact, they produce much less 

 than 50; therefore Castle's pure stock goes on in the 

 intermediate condition, and only by selection can he 

 pile up the factors and reach either extreme. 



Thus far the explanation seems satisfactory. Castle 

 showed, however, that the coat pattern condition 

 behaved in crosses as a simple Mendelian unit; that is, 

 it did not split up into complex ratios, but came out as 

 a recessive in a regularly 3 : i ratio. This really involves 

 no difficulty. Suppose Castle crosses one of his pure 

 strain rats having the pattern character with another 

 strain having an inhibitory factor for the pattern or some 

 other character that conceals the pattern. Since the 



