In 1844, and again in 1846, he published works on the geology of the voyage of the _Beagle_; the first on the Volcanic Islands visited, the second on South America. A second edition, in which both were combined in a single work, appeared in 1876. He seemed somewhat disappointed at the small amount of attention they at first attracted, and wrote with much humour to J. M. Herbert:--“I have long discovered that geologists never read each other’s works, and that the only object in writing a book is a proof of earnestness, and that you do not form your opinions without undergoing labour of some kind.” All geologists were, nevertheless, soon agreed in attaching the highest value to these researches. [Sidenote: ON CIRRIPEDES.] From this time forward his work was almost exclusively zoological. The four monographs on the Cirripedia, recent and fossil, occupied eight years--from October, 1846, to October, 1854. The works on the recent forms were published by the Ray Society (1851 and 1854), and those on the fossil forms by the Palæontographical Society (1851 and 1854). These researches grew directly out of his observations on the _Beagle_, but it is evident that they reached far greater dimensions than he had at first intended. Thus, at the very beginning of the work, he wrote (October, 1846) to Hooker:-- “I am going to begin some papers on the lower marine animals, which will last me some months, perhaps a year, and then I shall begin looking over my ten-year-long accumulation of notes on species and varieties, which, with writing, I dare say will take me five years, and then, when published, I dare say I shall stand infinitely low in the opinion of all sound Naturalists--so this is my prospect for the future.” Darwin himself, at any rate towards the end of his life, when he wrote his “Autobiography,” doubted “whether this work was worth the consumption of so much time,” although admitting that it was of “considerable value” when he had “to discuss in the ‘Origin of Species’ the principles of a natural classification.” Sir Joseph Hooker remembers that Darwin at an earlier time “recognised three stages in his career as a biologist: the mere collector at Cambridge; the collector and observer in the _Beagle_ and for some years afterwards; and the trained naturalist after, and only after, the Cirripede work” (Letter to F. Darwin).