10 THE FERTILISATION OF FLOWERS. [PART I. 
Insects which visit the different kinds of flowers by turns in the 
same way, must, since the same parts of their bodies always touch 
organs at the same height in the flower, effect cross-fertilisation 
both in dimorphic and trimorphic plants in such a way that 
the stigma in one form of flower always receives pollen from 
anthers which stand at the same height in another form. Such 
crossing, which is the mode commonly occurring in nature and in 
which the size of the pollen-grain is always proportional to the 
length of the style that its tube has to traverse, was called by 
Darwin legitimate. In dimorphic heterostyled plants there are 
thus two ways in which legitimate fertilisation is possible, both 
occurring regularly in nature, viz., the fertilisation of long-styled 
flowers with the pollen of short-styled, and vice versdé ; similarly 
there are in such cases two kinds of illegitimate fertilisation, viz., 
long-styled flowers with the pollen of long-styled, and short-styled 
with the pollen of short-styled. In trimorphic heterostyled 
plants, on the other hand, six modes of legitimate cross-fertilisa- 
tion are possible and occur regularly in nature; and there are 
twelve modes of illegitimate crossing, since each of the three 
kinds of stigmas is illegitimately fertilised if it receives pollen 
from either whorl of stamens in a flower of its own form or from 
one whorl of stamens in each of the other two. Now Darwin 
found, when he tried all the four modes of crossing in the 
dimorphic plants and all the eighteen modes in the trimorphic 
plants, and sowed the seeds from the resulting capsules and 
again crossed the offspring in various ways, that only the “ legiti- 
mate” crossings resulted in full fertility, and produced normal 
and fully fertile offspring; while, on the other hand, illegitimate 
crossings led to all degrees of diminished fertility or even 
complete barrenness, and produced offspring which had all the 
characters of bastards produced by the union of distinct species. 
The result of these investigations was particularly favourable to 
Knight's law, since it proved that in heterostyled plants not only 
the occasional crossing of separate flowers, but the regular crossing 
of separate individuals was absolutely essential for the maintenance 
of the species. At the same time it broke down the sharp 
boundary-line between Species and Variety which had formerly 
been supposed to be found in the more or less complete sterility of 
hybrids produced by crossing distinct species; and it showed, more- 
over, by the complete resemblance between the offspring of 
illegitimate unions in dimorphic and trimorphiec plants and the 
bastard offspring of distinct species, that in the latter sterility both 
