Among his contemporary men of science Darwin could at first count few converts. Hooker, whose candid and valuable criticisms of his friend's work had been continued up to the very end during its composition, did an eminent service to the cause of Evolution by publishing, almost simultaneously with the Origin of Species, his splendid memoir on The Flora of Australia, its Origin, Affinities, and Distribution, in which similar views were, not obscurely, indicated. Of Lyell, Darwin's other friend and counsellor, Huxley justly says:
'Lyell, up to that time a pillar of the antitransmutationists (who regarded him, ever afterwards, as Pallas Athene may have looked at Dian, after the Endymion affair), declared himself a Darwinian, though not without putting in a serious caveat. Nevertheless, he was a tower of strength and his courageous stand for truth as against consistency, did him infinite honour[139].'
Huxley himself accepted the theory of Natural Selection—but not without some important reservations—these, however, did not prevent him from becoming its most ardent and successful champion. Darwin used to acknowledge Huxley's great service to him in undertaking the defence of the theory—a defence which his own hatred of controversy and the state of his health made him unwilling to undertake—by laughingly calling him 'my general agent!' while Huxley himself in replying to the critics, declared that he was 'Darwin's bulldog.'
Although, at first, Darwin was able to enumerate less than a dozen naturalists who were prepared to accept his views, while influential leaders of thought in science—like Richard Owen in this country and Louis Agassiz in America—were bitterly opposed to them, the theory gradually obtained supporters especially among the younger cultivators of botany, zoology and geology.
It is evident that Darwin for some time regarded his 'abstract,' as he called the Origin of Species, as only a temporary expedient—one to be superseded by the publication of the much more extended work, designed and commenced long before. Although the Origin was only published late in November 1859, and he was called upon immediately to prepare a second edition, we find that on January 1st, 1860, Darwin began to arrange his materials for dealing with the first great division of his subject, 'the variation of animals and plants under domestication.' So numerous and important were his notes and records of experiments, however, that he soon found that to expand the whole of the 'abstract,' on the same scale, would be an impossible task for any one man, however able and diligent. Unwilling that the results of some of his special 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 between their mental and moral processes and their modes of exhibiting them and our own; while visits were made by him to the Zoological Gardens with the same object. By reading and correspondence also, an enormous mass of notes was collected, and on February 4th, 1868, having seen his great work on Variation under Domestication published, Darwin was able to make the entry in his diary, 'Began work on Man.'
As was usual with most of his works, Darwin underestimated the time required to complete it. Through all the years 1867—'68, '69 and '70 we find the entries in his diary 'working at Descent of Man,' and only early in the year 1871 was the book finished. His original plan of compressing his notes on the expression of the Emotions into a chapter at the end of the book proved to be impracticable, and the material was reserved for a new work. This work, The Expression of the Emotions in Man and Animals, was commenced directly the Descent of Man was out of hand, a rough copy was finished by April 27th, 1871, but the last proofs were not corrected till August 23rd, 1873.
In dealing with the question of the origin of the human race, Darwin was led to propound his views concerning Sexual selection, the results of the preferences shown by males and females, respectively, not only among mankind, but in various other animals. It was with respect to some of the conclusions contained in this work that Wallace found himself unable to follow Darwin. Wallace maintained that while man's body could have been developed by Natural Selection, his intellectual and moral nature must have had a different origin. He also declined to adopt the theory of sexual selection, so far as it depends on preferences exhibited by females for beauty in the males. Wallace, however, in some respects has always been disposed to attach more importance to Natural Selection, as the greatest, if not the only factor in evolution, than Darwin himself.