18 



The long stem (Bb) in 87 plants. 

 The short stem (b) in 79 plants. 



The theory adduced is therefore satis- 

 factorily confirmed in this experiment 

 also. 



For the characters of form of pod, 

 colour of pod, and position of flowers, 

 experiments were also made on a small 

 scale, and results obtained in perfect 

 agreement. All combinations which 

 were possible through the union of the 

 differentiating characters duly ap- 

 peared and in nearly equal numbers. 



Experimentally, therefore, the 

 theory is confirmed that the pea hy- 

 brids form egg a?id pollen cells which, 

 in their coTistitiition, represent in eqiial 

 numbers all constant forms which re- 

 sult from the combinatioji of the char- 

 acters united iji fertilisation. 



The difference of the forms among 

 the progeny of the hybrids, as well as 

 the respective ratios of the numbers in 

 which they are observed, find a suffi- 

 cient explanation in the principle above 

 deduced. The simplest case is afforded 

 by the developmental series of each 

 pair of differentiating characters. This 

 series is represented by the expression 

 A + lAa + a, in which A and a sig- 

 nify the forms with constant differ- 

 entiating characters, and Aa the hybrid 

 form of both. It includes in three dif- 

 ferent classes four individuals. In the 

 formation of these, pollen and t^^ cells 

 of the form A and a take part on the 

 average equally in the fertilisation; 

 hence each form [occurs] twice, since 

 four individuals are formed. There 

 participate consequently in the ferti- 

 lisation 



The pollen cells A + A + a + a 

 The egg cells A + A + a + a. 



It remains, therefore, purely a mat- 

 ter of chance which of the two sorts 

 of pollen will become united with each 



MENDEL 



separate egg cell. According, however, 

 to the law of probability, it will always 

 happen, on the average of many cases, 

 that each pollen form, A and a, will 

 unite equally often with each egg cell 

 form, A and a, consequently one of the 

 two pollen cells A in the fertilisation 

 will meet with the egg cell A and the 

 other with an egg cell a, and so like- 

 wise one pollen cell a will unite with 

 an egg cell A, and the other with egg 

 cell a. 



The result of the fertilisation may be 

 made clear by putting the signs of the 

 conjoined egg and pollen cells in the 

 form of fractions, those for the pollen 

 cells above and those for the egg cells 

 below the line. We then have 



j4 /i _a_ _a_ 

 A I' A a ' 



In the first and fourth term the egg 

 and pollen cells are of like kind, con- 

 sequently the product of their union 

 must be constant, viz. A and a; in the 

 second and third, on the other hand, 

 there again results a union of the two 

 differentiating characters of the stocks, 

 consequently the forms resulting from 

 these fertilisations are identical with 

 those of the hybrid from which they 

 sprang. There occurs accordingly a 

 repeated hybridisation?. This explains 

 the striking fact that the hybrids are 

 able to produce, besides the two pa- 

 rental forms, offspring which are like 



themselves; — and — r- both give the 

 'a A ^ 



same union Aa, since, as already re- 

 marked above, it makes no difference 

 in the result of fertilisation to which 

 of the two characters the pollen or 

 egg cells belong. We may write then 



