MENDEL'S LIFE AND WORK 27 



the slightest approach to the opposite character 

 with which it had been so intimately associated, 

 was a fact of fundamental importance, and Mendel 

 seized upon it Avith characteristic acimien as the 

 key to the whole position. He asked himself the 

 question, " How is it possible for the original 

 characters to come out again in the progeny of 

 hybrids in an absolutely pure form ? " His 

 answer "was, since we know from experience 

 that to keep a race constant it is essential that 

 only individuals exhibiting the same character 

 should be mated together, so when constant 

 forms arise from hybrids it ihust be due to the 

 fact that the}' are produced b}' the meeting of 

 germ-cells containing the factor for the character 

 in question and that alone. 



Having arrived at the conclusion that in the 

 formation of some at any rate of the germ-cells 

 of hybrids a complete segregation of character- 

 factors takes place, Mendel next sought for an 

 explanation of the fact that, in regard to each 

 pair of characters, the number of pure forms 

 arising from the hybrids was exactly one-half of 

 the total number produced (^D + ]R). With 

 his mathematical training he had no difficulty in 

 seeing that the only assumptions necessary to 

 account for this are that the segregation of 

 character-factors should take place in such a way 

 that half the germ-cells (whether male or female) 

 carry the factor for one character and half for 

 the other, and that the matings of the two types 



