A DEFINITION OF EVOLUTION 



1841. His closest associate during this time was Lyell, who perhaps has 

 contributed more than any other man to the modernization of geology. 

 But his health became progressively worse, and he was unable to bear 

 much excitement, and so the Darwins moved to Down, a country resi- 

 dence, in 1842. It was here that most of his life's work was done. As his 

 health forced him to remain in seclusion, the remainder of his biography 

 becomes largely a catalogue of his books. Much evidence indicates that 

 his problems were largely psychosomatic. 



In 1842, Darwin published the first of his major geological works result- 

 ing from the voyage of the Beagle, "The Structure and Distribution of 

 Coral Reefs." In this book, he presented a theory of the structure and 

 mode of formation of coral reefs which was very different from the one 

 then generally accepted. But Darwin's keen observations and accurate 

 thinking on the subject won support, and his theory is even now generally 

 accepted among geologists. This was followed in 1844 by "Geological 

 Observations on Volcanic Islands" and in 1846 by "Geological Observa- 

 tions on South America." 



In 1846, Darwin began work on a study of the Cirripedia, or barnacles. 

 This began with the study of an aberrant barnacle, which burrows into 

 the shells of other species, collected when the Beagle visited the coast of 

 Ghile. In order to understand the structure of this new species, it was 

 necessary to dissect more typical forms. Gradually the scope of the study 

 broadened until it included descriptions of all known species of barnacles, 

 living and fossil. This great Monograph on the Girripedia was published 

 in four volumes. The Ray Society published two volumes on the living 

 Cirripedia in 1851 and 1854, respectively, while the Palaeontological So- 

 ciety published the two volumes on fossil species in the same years. Of 

 this work, Darwin said, "1 do not doubt that Sir E. Lytton Bulwer had me 

 in his mind when he introduced in one of his novels a Professor Long, 

 who had written two huge volumes on limpets. . . . My work on the Gir- 

 ripedia possesses, I think, considerable value, as besides describing several 

 new . . . forms, I made out the homologies of the various parts . . . and 

 proved the existence in certain genera of minute males. . . . The Girripedia 

 form a highly varying and difficult group of species to class; and mv work 

 was of considerable use to me, when I had to discuss in the 'Ori<j;in of 

 Species' the principles of natural classification. Nevertheless, I doubt 

 whether the work was worth the consumption of so much time." Yet Sir 

 Joseph f looker, a distinguished botanist, wrote to one of Darwin's sons 

 that "Your father recoirni/ed three stages in his career as a biolotiist: 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, th(> Cirripede work. " T. H. Huxley seems to haxe concurred in this 

 opiiu'oii. 



The Origin of Species. No further books followed until the "Origin of 

 Species " in 1859. Yet this had really been in the making for more* than 

 twenty years. During the voyage of the Beagle, various facts of paleon- 

 tology and biogeographv which Darwin observed had suggested to him 

 the possibility that species might not be immutable. But he had no theory 



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