All this shows how certain he felt that he was on firm ground, and that his theory of natural selection was of vast importance to science. This same strong conviction appears clearly in the first edition of the “Origin,” and is undoubtedly one of the secrets of its power to move the minds of men. Although the author is above all others fair-minded; although he is most keen to discover and to bring forward all opposing evidence, and to criticise most minutely everything favourable; nevertheless, looking at the evidence as a whole, he has no doubt as to its bearing, and feels, and shows that he feels, a magnificent confidence in the truth and the importance of his theory. CHAPTER VII. GROWTH OF THE “ORIGIN” (_continued_)--CORRESPONDENCE WITH FRIENDS. The great periods of Darwin’s scientific career are marked by intimate friendships, which must be taken into account in attempting to trace his mental development. Henslow was his intimate friend at Cambridge and during the voyage of the _Beagle_. The influence of Lyell, through his writings, was of the utmost importance during the voyage, and was deepened by the close personal contact which took place on Darwin’s return. Sir Joseph Hooker was his most intimate friend during the growth of the “Origin of Species.” Although Hooker met Darwin in 1839, their friendship did not begin until four years later, when the former returned from the Antarctic Expedition. On January 11th, 1844, Darwin wrote admitting his conclusions on the question of evolution:--“At last gleams of light have come,” he says, “and I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing a murder) immutable” (“Life and Letters,” Vol. II. p. 23). [Sidenote: INQUIRIES AND EXPERIMENTS.] From this point onwards his letters, especially to Hooker, indicate the course he was following and the various problems he was considering as they arose. Thus we find that he had finished reading Wollaston’s “Insecta Maderensia” in 1855 (writing March 7th), and had been struck with the very large proportion of wingless beetles, and had interpreted the observation, viz. “that powers of flight would be injurious to insects inhabiting a confined locality, and expose them to be blown to the sea.” It is of great interest thus to witness the origin of a theory which has since been universally accepted, and has received confirmation from many parts of the world. On April 11th of the same year he is experimenting on the powers of resistance to immersion in salt water possessed by seeds, and he writes an account of it to Hooker. The object of these experiments was to throw light on the means by which plants have been transported to islands.