74 Mendel's Experiments 



The result of the fertilisation may be made clear by 

 putting the signs for 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 



A A a a 



- + - - + + -. 

 A a A a 



In the first and fourth term the egg and pollen cells are of 

 like kind, consequently 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 different- 

 iating 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 parental forms, offspring which are like themselves ; 



and both give the same union A a, since, as already 



remarked 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 



A A a a 



- + - + r + - = 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 solitary flowers and in solitary 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 egg 

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

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



* [Whether segregation by such units is more than purely for- 

 tuitous could probably be determined by seriation.] 



