Besides revision in detail, there was of course always a tendency to larger modifications of the general theory. Ingenious and far-reaching as natural selection was, the difficulties connected with it were so immense, that the loyalty of even its discoverer at times necessarily wavered, or perhaps we should say better, his enthusiasm heated and cooled. Moreover, natural selection depended upon variation, and variation to Darwin was always an inexplicable puzzle, for which no solution or too many might be found. Disinclined as he was to accept or even to respect his predecessors, Buffon and Lamarck, Darwin in later years, when the pressure on natural selection became fiercer, seemed to turn more to the adaptive solutions of these predecessors. As Professor Morgan puts it: ‘Despite the contempt with which Darwin referred to Lamarck’s theory, he himself, as we have seen, often made use of the principle of the inheritance of acquired characters, and even employed the same illustrations cited by Lamarck.’[376] And Professor Osborn indicates admirably the gradual process of the change which took place in Darwin’s attitude: ‘Starting with some leaning towards the theories of modification of Buffon and Lamarck, he reached an almost exclusive belief in his own theory, and then gradually inclined to adopt Buffon’s and then Lamarck’s theories as well, until in his maturest writings he embraced a threefold causation in the origin of species.’[377] The drift towards Lamarck is well shown in a passage of a letter to Galton, written in 1875: ‘If this implies that many parts are not modified by use and disuse during the life of the individual, I differ widely from you, as every year I come to attribute more and more to such agency.’[378] At the same time, to the very end natural selection remained in Darwin’s mind not only the quintessence of his theorizing, but the prime agent by which modification had been accomplished. In ‘The Descent of Man’ he explains and in a manner excuses any earlier undue insistence upon it, but also reiterates his firm faith in its great, if not omnipotent efficacy: ‘If I have erred in giving to natural selection great power, which I am very far from admitting, or in having exaggerated its power, which is in itself probable, I have at least, as I hope, done good service in aiding to overthrow the dogma of separate creations.’[379] VII