It is well known that during Darwin’s voyage in the Beagle he was deeply impressed by discovering extinct armadillo-like fossil forms in South America, the home of armadilloes, and by observing the relationship of the plants and animals of each island in the Galapagos group to those of the other islands and of South America, the nearest continent.  These facts suggested evolution, and without evolution appeared to be meaningless.

Evolution and its motive cause were the problems which “haunted” him for the next twenty years.  The first step towards a possible solution was the “opening of a notebook for facts in relation to the origin of species” in 1837, two years before the publication of his Journal.  From the very commencement of his literary and scientific work, a rule rigidly adhered to was that of interspersing his main line of thought and research by reading books touching on widely diverging subjects; and it was thus, no doubt, that during October, 1838, he read “for amusement” Malthus’s “Essay on Population”; not, as he himself affirms, with any definite idea as to its intimate bearing on the subject so near his heart.  But the immediate result was that the idea of Natural Selection at once arose in his mind, and, in his own words, he “had a theory by which to work.”

In May and June, 1842, during a visit to Maer and Shrewsbury, he wrote his first “pencil sketch of Species theory,” but not until two years later (1844) did he venture to enlarge this to one of 230 folio pages, “a wonderfully complete presentation of the arguments familiar to us in the ‘Origin.’"[19]

Already, in addition to the mass of facts collected, Darwin was busy with some of the experiments which he described in a letter to Sir Joseph Hooker (in 1855) as affording the latter a “good right to sneer, for they are so absurd, even in my opinion, that I dare not tell you.”  While a sentence in another letter (dated 1849) throws a sidelight on all this preparatory work:  “In your letter you wonder what ‘ornamental poultry’ has to do with barnacles; but do not flatter yourself that I shall not yet live to finish the barnacles, and then make a fool of myself on the subject of species, under which head ornamental poultry are very interesting.”

Somewhere about this time (1842-44), Darwin, referring to the idea of Natural Selection which arose in his mind after reading Malthus on “Population” four years earlier, continues:  “But at that time I overlooked one problem of great importance ... the tendency in organic beings descended from the same stock to diverge in character as they become modified ... and I can remember the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me....  The solution, as I believe, is that the modified offspring of all dominant and increasing forms tend to become adapted to many and highly diversified places in the economy of nature."[20]

So convinced was he of the truth of his ideas as expressed in the 1844 MS., that immediately after its completion he wrote the memorable letter to Mrs. Darwin telling her what he would wish done regarding its publication in the event of his death.