cial
researches should be lost, he wisely determined to issue them as
separate books. The first of these to appear was that on the
_Fertilisation of Orchids_, a beautiful illustration of the relation of
insects to flowers in producing crossing. He had been more than twenty
years working and experimenting on this subject, his interest in it
having been quickened by having read an almost forgotten book of the
botanist Sprengel. Almost at the same time, and in following years, he
wrote papers for the Linnean Society on dimorphic and trimorphic forms
of flowers, and their bearing on the question of cross-fertilisation.
These papers were the foundation of his well-known work, _The Different
Forms of Flowers on Plants of the same Species_. In the same way, a
paper read in 1864 to the Linnean Society was subsequently expanded into
_The Movements and Habits of Climbing Plants_.

Owing to delays caused by the preparation and publication of these books
and frequent interruptions from sickness, the work on variation did not
appear till 1868. It was a very extensive piece of work in two volumes,
and, at its end, Darwin tentatively propounded a hypothesis to account
for the facts of Heredity and Variation to which he gave the name of
'pangenesis.'

Charles Darwin had reached the age of fifty, when he wrote the _Origin
of Species_. At a very early period in his career, he had resolved that
he would never start a new theory or revise an old one after he was
sixty; as he used laughingly to say, 'I have seen too many of my friends
make fools of themselves by doing that.' But as he approached this
'fatal age,' one more subject of a theoretical and highly controversial
nature remained to be dealt with, namely, the question of the
application of the theory of natural selection to man, both as regards
his physical structure and his intellectual and moral characteristics.

Darwin tells us that in 1837 or '38, as soon as he had become 'convinced
that species were mutable productions,' he 'could not avoid the belief
that man must come under the same law[140].' From that time, he began
collecting facts bearing on the question. As each of his children was
born, he examined closely the signs of dawning intelligence, and made
notes of the manner in which new sensations and passions were exhibited
by them. His dog and other animals, for whom he always showed the
greatest fondness, were closely watched with the object of noting
correspondences be