in Hybridisation 363 



the two parental forms, offspring which are Hke themselves; 



A a . 



— and —. both give the same union Aa, since, as already 



(t y^t 



remarked above, it makes no difference in the result of 



fertilisation to which of the two characters the pollen or 



^Z% cells belong. We may write then 



A A a a ^ , 



-77 + — + -;^ ^- = A-^2Aa^a, 



A a A a 



This represents the average result of the self-fertilisation 

 of the hybrids when two differentiating characters are united 

 in them. In individual flowers and in individual plants, 

 however, the ratios in which the forms of the series are pro- 

 duced may suffer not inconsiderable fluctuations'* Apart 

 from the fact that the numbers in which both sorts of ^<g^ 

 cells occur in the seed vessels can only be regarded as equal 

 on the average, it remains purely a matter of chance which 

 of the two sorts of pollen may fertilise each separate ^"g^ 

 cell. For this reason the separate values must necessarily 

 be subject to fluctuations, and there are even extreme cases 

 possible, as were described earlier in connection with the 

 experiments on the form of the seed and the colour of the 

 albumen. The true ratios of the numbers can only be 

 ascertained by an average deduced from the sum of as 

 many single values as possible ; the greater the number 

 the more are merely chance effects eliminated. 



The developmental series for hybrids in which two 

 kinds of differentiating characters are united contains among 

 sixteen individuals nine different forms, viz. : 



AB ^ Ab ^ aB ^ ab ^ lABb ^ 2aBb ^ 2AaB -\- 2Aab ^ 



^AaBb. 



Between the differentiating characters of the original stocks 

 Aa and Bb four constant combinations are possible, and 

 consequendy the hybrids produce the corresponding four 

 forms of ^g<g and pollen cells AB, Ab, aB, ab, and each 

 of these will on the average figure four times in the 



* [Whether segregation by such units is more than purely fortuitous 

 may perhaps be determined by seriation.] 



