THE GENOTYPICAL RESPONSE OF THE PLANT 343 
maintain that the problems connected with the formation of the 
Linnean species are exhausted by this demonstration. Most of these 
species are, as every earnest inquirer will find, in their natural areas 
of distribution rather circumscribed products, which do not live in 
any extensive connubium with congeners of other species. The brid- 
geless gaps found between species of the same genus, the final moul- 
ding of the Linnean species, remain then to be explained. The Dar- 
winian idea of selective processes at present offers to most minds a 
plausible explanation of the differentiation of Linnean species. 
Although very little is known with regard to the actual play of these 
selective processes, certain facts likely to demonstrate the complex 
nature of selection have been brought to light. KôLrEUTER (1761) 
showed that a species pollinated simultaneously with its own pollen 
and pollen from another species breeds’ true to type, in spite of the 
fact that it otherwise gives hybrids when crossed with that species. 
That the native pollen is favoured as compared with foreign has 
been shown by HERIBERT-NiLssoX (1920) in the case of Oenothera 
Lamarckiana. He found that pollen tubes of O. gigas grew more 
slowly in the styles of O. Lamarckiana than the O. Lamarckiana’s 
own pollen tubes. The terms elimination, certation, prohibition, and 
substitution discussed by HERIBERT-NiLsson refer to phenomena which 
give rise to aberrant types of segregation. The importance of such 
gametic and zygotic complications has been discussed more recently 
by Nizssox-EuLe (1921). They are all particularly well calculated to 
throw light upon selective processes: of great weight. To whatever 
extent this »pre-natal selection» may limit the output of new or- 
ganisms, hybrids between already existing species would no doubt 
be more numerous and more widely distributed in nature were it 
not for the controlling effect of living and non-living factors of the 
outer world. Various disturbances involving different organs. are 
frequently seen in hybrids and in »artificial» species, and this fact 
does not support the idea that such organisms are able to hold their 
own with nature. We are thus forced to the conclusion that the 
present-day species represent the necessary outcome of the complex 
processes of selection in this epoch of the earth’s history (cp. HERIBERT- 
Nizssox, 1918). As a natural consequence we are led to the inference 
that a change in the non-living world brings about a corresponding 
change in the living, inducing a recombination of Mendelian factors 
now distributed in organisms, and resulting in the formation of new 
genotype compounds or species (= evolution). 
