The fourth edition of "The Origin," in 1866, was longer, by fifty pages, than its predecessor. Among the additions may be mentioned a fuller treatment of the argument from embryology, which was made stronger by later investigations. The fifth edition (1869) was comparatively little increased in bulk, though altered in many details. In particular it contained a somewhat important change relating to the extent of the influence of natural selection. This is also referred to in "The Descent of Man" (first edition, vol. i. pp. 152-3), where the author says he had not formerly considered sufficiently the existence of many structures which appeared to be neither beneficial nor injurious, and had attributed too much to natural selection. "I was not able," he says, "to annul the influence of my former belief, then widely prevalent, that each species had been purposely created; and this led to my tacitly assuming that every detail of structure, excepting rudiments, was of some special, though unrecognised, service.... If I have erred in giving to natural selection great power, which I am 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." The sixth edition (1872), in smaller type, was considerably revised and altered, and remains permanent. A glossary of scientific terms was added by Mr. W. S. Dallas. A new chapter was inserted after the sixth, and entitled "Miscellaneous Objections to the Theory of Natural Selection." It was partly derived from modified portions of chapter iv. of former editions, but the latter and larger part was new, and relates chiefly to the supposed incompetency of natural selection to account for the very early stages of useful structures. Numerous cases, such as the development of the giraffe's neck, the baleen of the whale, the mammary glands, &c., are admirably discussed. Causes preventing the acquisition, through natural selection, of useful structures in many cases are dealt with, and reasons given for disbelieving in great and sudden modifications. In the concluding chapter Darwin further admits that he had formerly underrated the frequency and importance of use and disuse of parts, of the direct action of external conditions, and of variations which seem to us, in our ignorance, to arise spontaneously. He alludes to misrepresentations of his views, and calls attention to the fact that, in the first edition, at the close of the introduction, he stated his conviction that natural selection had been the main, _but not the exclusive_ means of modification. "This has been of no avail. Great is the power of steady misrepresentation; but the history of science shows that, fortunately, this power does not long endure." This is Darwin's almost sole allusion in his works to the persistence with which views not his had been attributed to him, or he had been calumniated for views he did hold.