d by this observation, as he had expected a
prejudicial influence of self-fertilisation to manifest itself after
a series of generations: "I always supposed until lately that no
evil effects would be visible until after several generations of
self-fertilisation, but now I see that one generation sometimes suffices
and the existence of dimorphic plants and all the wonderful contrivances
of orchids are quite intelligible to me." ("More Letters", Vol. II. page
373.)

The observations on Linaria and the investigations of the results of
legitimate and illegitimate fertilisation in heterostyled plants were
apparently the beginning of a long series of experiments. These were
concerned with plants of different families and led to results which are
of fundamental importance for a true explanation of sexual reproduction.

The experiments were so arranged that plants were shielded from
insect-visits by a net. Some flowers were then pollinated with their own
pollen, others with pollen from another plant of the same species. The
seeds were germinated on moist sand; two seedlings of the same age, one
from a cross and the other from a self-fertilised flower, were selected
and planted on opposite sides of the same pot. They grew therefore under
identical external conditions; it was thus possible to compare their
peculiarities such as height, weight, fruiting capacity, etc. In other
cases the seedlings were placed near to one another in the open and in
this way their capacity of resisting unfavourable external conditions
was tested. The experiments were in some cases continued to the tenth
generation and the flowers were crossed in different ways. We see,
therefore, that this book also represents an enormous amount of most
careful and patient original work.

The general result obtained is that plants produced as the result of
cross-fertilisation are superior, in the majority of cases, to those
produced as the result of self-fertilisation, in height, resistance to
external injurious influences, and in seed-production.

Ipomoea purpurea may be quoted as an example. If we express the result
of cross-fertilisation by 100, we obtain the following numbers for the
fertilised plants.

Generation. Height. Number of seeds.

1 100: 76 100: 64
2 100: 79 -
3 100: 68 100: 94
4 100: 86 100: 94
5 100: 75 100: 89
6 100: 72