154 THE OLDER HYBRIDISTS 



One of the most noted of Kolreuter's experiments 

 was that which consisted in repeatedly recrossing a 

 hybrid plant with one of the parent species from which 

 the hybrid was derived. By continuing to pollinate 

 the members of one generation after another with the 

 pollen of the same parent species, plants were at last 

 arrived at which were indistinguishable from the parent 

 in question. We shall return to this fact later on, 

 when the reader will be in a position to appreciate its 

 importance more fully. 



Kolreuter found that the result of reciprocal crosses 

 is usually identical that is to say, the offspring ob- 

 tained by fertilizing a plant A with pollen from a 

 plant B are not to be distinguished from those ob- 

 tained when B is fertilized with the pollen of A. But 

 the two opposite processes of fertilization are not 

 always equally easy to carry out. An extreme instance 

 of this circumstance was met with in the case of the 

 genus Mirabilis. Mirabilis jalapa was easily fertilized 

 with pollen from M. longiflora. During eight years 

 Kolreuter made more than two hundred attempts to 

 effect the reverse cross, but without success. 



It was shown by Kolreuter that hybrids between 

 different races or varieties of the same species are 

 usually much more fertile than hybrids obtained by 

 crossing distinct species. Indeed, he believed that 

 varieties of a single species were in all cases perfectly 

 fertile together, whilst hybrids between species always 

 showed some degree of sterility. But in this case Kol- 

 reuter based his definition of a species upon the very 

 point at issue, and when he found forms, which other 



