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. It will be seen that although Darwin had in all probability thought out all his important theoretical conclusions before 1869, when he reached the 'fatal age,' yet, owing to various delays, the books, in which he embodied his views, had not all appeared till more than four years later. Lyell, who was a convinced evolutionist before the publication of the _Principles of Geology_, as is shown by his letters,--and the fact is strongly insisted on both by Huxley and Haeckel[141],--was slow in coming into _complete_ agreement with Darwin concerning the theory of Natural Selection. While he followed his friend's investigations with the deepest interest, his less sanguine nature led him often to despair of the possibility of solving 'the mystery of mysteries.' As Darwin wrote only a year before his own death, Lyell 'would advance all _possible_ objections to my suggestions, and _even after these were exhausted_ would long _remain dubious_[142].' It is evident from the correspondence that Darwin was at times tempted to become impatient with the friend, for whose advocacy of his views he so deeply longed. Fourteen years after the publication of the _Origin of Species_, however, Lyell, in his _Antiquity of Man_, gave in his adhesion to Darwin's theory but, even then, not in the unqualified manner that the latter desired. Yet I have reason to know that some years before his death, Lyell was able to assure his friend of his _complete_ agreement, and Darwin, six years after the loss of his friend, wrote, 'His candour was highly remarkable. He exhibited this by becoming a convert to the Descent theory, though he had gained much fame by opposing Lamarck's views, _and this after he had grown old_.' Darwin adds that Lyell, referring to the '_fatal_ age' of sixty, said 'he hoped that now he might be allowed to live[143]!'