THE EXPERIMENTAL STUDY OF GENETIC RELATIONSHIPS I5I 
We can hardly fail to agree that Darwin was right when he said 
that defective pollen could arise within a single well-circumscribed 
species as a result of illegitimate pollination. We now know, thanks 
to the investigations of Bateson and Gregory, that heterostylism in 
Primula is inherited as a simple Mendelian character. Legitimate 
pollination maintains heterozygosis: illegitimate pollination, on the 
contrary, leads to the extraction of dominants which, in a sense, are 
genetically "purer" than the heterozygotes. Nevertheless, they may 
show, apparently as a result of their increased "purity," the very 
character of the pollen which Bateson and Jeffrey consider a sign of 
hybridity. A further interesting fact relating to Primula is as follows: 
strains of P. sinensis sometimes throw a mutation in which style and 
anthers are of the same length. This form, called equal-styled by 
Darwin, has been shown to be recessive to the long-styled form, which, 
in turn, is recessive to the short-styled form. Darwin cultivated 
several equal-styled races. He tells us that "my son, Mr. W. E. 
Darwin, . . . examined pollen from two equal-styled plants which 
he procured at Southampton ; and in both the grains differed extremely 
in size, a large number being small and shrivelled, whilst many were 
fully as large as those of the short-styled form and rather more glo- 
bular . . . The vast number of the small and shrivelled grains in the 
above two cases explains the fact that though equal styled plants are 
usually fertile in a high degree, yet some yield few seeds." Darwin tells 
us that his equal-styled races came true from seed, as, being extreme 
recessives, they must of course have done. Again we have a case of 
great genetic purity in association with defective pollen. There is no 
evidence, according to Bateson, that Primula sinensis has ever been 
hybridized. It seems to be one of the few cultivated plants in which 
great diversity has come about without any admixture with other 
species, although its purity is not as well attested as that of the sweet 
pea. 
Examples might be multiplied indefinitely which show that de- 
fective pollen is as likely to indicate mutation as hybridization. In 
fact, I believe that it may be laid down as a rule that both processes 
are generally characterized by pollen sterility. With this conclusion 
in mmd we may judge the mutation theory with a better chance of 
arriving at an unbiased decision. 
Why is it that the polymorphic groups in which mutation is taking 
place, or supposed to be taking place, all show such undoubted evi- 
