MENDEL 



4-+ -+ ■^+-^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, how- 

 ever, the ratios in which the forms of 

 the series are produced may suffer not 

 inconsiderable fluctuations.^^ Apart 

 from the fact that the numbers in 

 which both sorts of egg 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 egg 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 de- 

 duced from the sum of as many single 

 values as possible; the greater the num- 

 ber, the more are merely chance effects 

 eliminated. 



The developmental series for hy- 

 brids in which two kinds of differen- 

 tiating characters are united contains, 

 among sixteen individuals, nine differ- 

 ent forms, viz., 



AB + Ab + aB + ab + lABb + laBb 

 + lAaB -f 2Aab + '\AaBb. 



Betu^een the differentiating characters 

 of the original stocks, Aa and Bb, four 

 constant combinations are possible, and 

 consequently the hybrids produce the 

 corresponding four forms of egg and 

 pollen cells AB, Ab, aB, ab, and each 



19 



of these will on the average figure four 

 times in the fertilisation, since sixteen 

 individuals are included in the series. 

 Therefore the participators in the fer- 

 tilisation are 



Pollen cells AB + AB + AB + AB 

 + Ab + Ab + Ab + Ab 

 + aB + aB + aB + aB 

 + ab + ab + ab + ab. 



Egg cells AB + AB + AB + AB 

 + Ab + Ab + Ab + Ab 

 + aB + aB + aB + aB 

 + ab + ab + ab + ab. 



In the process of fertilisation each pol- 

 len form unites on an average equally 

 often with each egg cell form, so that 

 each of the four pollen cells AB unites 

 once with one of the forms of egg cell 

 AB, Ab, aB, ab. In precisely the same 

 way the rest of the pollen cells of the 

 forms Ab, aB, ab unite with all the 

 other egg cells. We obtain therefore 



Ab 



or 



19 [Whether segregation by such units is 

 more than purely fortuitous may perhaps be 

 determined by seriation.] 



AB -h ABb + AaB 4- AaBb + ABb 

 + Ab + AaBb + Aab + AaB 

 + AaBb + aB + aBb + AaBb 

 + Aab + aBb + ab = AB + Ab 

 + aB + ab + lABb + laBb 

 + lAaB + lAab + 4AaBb.^ 



In precisely similar fashion is the de- 

 velopmental series of hybrids exhibited 

 when three kinds of differentiating 



20 [In the original the sign of equality (=:) 

 is here represented by +, evidently a mis- 

 print.] 



