Another instance of Darwin’s zeal and ingenuity in reasoning is the theory of sexual selection, devised to meet some difficulties in his general argument. Many animals have certain so-called secondary sexual characteristics, that is, characteristics affecting one sex only yet not directly involved in the reproductive process. Darwin believed that these characteristics were largely developed through the working of natural selection upon the basis of the preference of one sex for individuals of the other for mating purposes. That is to say, the splendor of the male peacock’s tail made him more attractive to the female and therefore more successful with her. There were numerous and obvious difficulties in this view, as the assumption of a fine æsthetic sense in comparatively lowly organized creatures, and it was energetically disputed from the start. Wallace, Darwin’s ardent fellow-thinker and coadjutor, was anything but favorable to it. Yet Darwin’s faith was never really shaken. He persisted to the end, making new experiments and investigations, and meeting his adversaries’ contentions with vast and varied resource. In ‘The Descent of Man’ he wrote: ‘For my own part I conclude that of all the causes which have led to the differences in external appearance between the races of man, and to a certain extent between man and the lower animals, sexual selection has been the most efficient.’[363] Nevertheless, it is fascinating to see him reveal the doubt and the questioning that were inwrought with conviction in his mind. He writes to Wallace: ‘I grieve to differ from you, and it actually terrifies me and makes me constantly distrust myself.’[364] Again: ‘You will be pleased to hear that I am undergoing severe distress about protection and sexual selection; this morning I oscillated with joy towards you; this evening I have swung back to my [old] position, out of which I fear I shall never get.’[365] And he sums up the process with a general remark of the largest and most fruitful bearing: ‘I sometimes marvel how truth progresses, so difficult is it for one man to convince another, unless his mind is vacant. Nevertheless, I myself to a certain extent contradict my own remark, for I believe far more in the importance of protection than I did before reading your articles.’[366]