KOLREUTER 165 
One of the most noted of Kélreuter’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. 
Koélreuter 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 
K6lreuter made more than two hundred attempts to 
effect the reverse cross, but without success. 
It was shown by Kélreuter 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 Kél- 
reuter based his definition of a species upon the very 
point at issue, and when he found forms, which other 
