With characteristic caution, Darwin determined not to write down 'even the briefest sketch' of this hypothesis, that had so suddenly presented itself to his mind. His habit of thought was always to give the fullest consideration and weight to any possible objection that presented itself to his own mind or could be suggested to him by others. Though he was satisfied as to the truth and importance of the principle of natural selection, there is evidence that for some years he was oppressed by difficulties, which I think would have seemed greater to him than to anyone else. In my conversations with Darwin, in after years, it always struck me that he attached an exaggerated importance to the merest suggestion of a view opposed to that he was himself inclined to adopt; indeed I sometimes almost feared to indicate a possible different point of view to his own, for fear of receiving such an answer as 'What a very striking objection, how stupid of me not to see it before, I must really reconsider the whole subject.'
While a divinity student at Cambridge, Darwin had been much struck with the logical form of the works both of Euclid and of Paley. The rooms of the latter he seems to have actually occupied at Christ's College and the works of the great divine were so diligently studied that their deep influence remained with him in after life[109].
I think it must have been the remembrance of the arguments of Paley on the 'proofs of design' in Nature, that seem in after life to have haunted Darwin so that for long he failed to recognise fully that the principle of natural selection accounted not only for the adaptation of an organism to its environment, but at the same time explains that divergence, which must have taken place in species in order to give rise to their wonderfully varied characters.
It was not till long after he came to Down in 1842, he tells us in his autobiography, that his mind freed itself from this objection. He says:—
'I can remember the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me,'
and he compares the relief to his mind as resembling the effect produced by 'Columbus and his egg[110].' Some may think the 'solution' of Columbus was itself not a very satisfactory one; and I am inclined to regard the difficulties of which Darwin records so sudden and dramatic a removal as more imaginary than real!
There can be no doubt that, as pointed out by the late Professor Alfred Newton[111], there was among naturalists during the second quarter of the nineteenth century a feeling of dissatisfaction with respect to current ideas concerning the origin of species, accompanied in many cases with one of expectation that a solution might soon be found. Others, however, despairingly regarded it as 'the mystery of mysteries' for which it was hopeless to attempt to find a key. There was, however, one man, who simultaneously with Darwin was meditating earnestly on the problem and who eventually reached the same goal.
Alfred Russel Wallace was born thirteen years after Darwin, and a quarter of a century after Lyell. He did not possess the moderate income that permits of entire devotion to scientific research—an advantage, the importance of which in their own cases, both Lyell and Darwin were always so ready to acknowledge. Wallace, after working for a time as a land-surveyor and then as a teacher, at the age of 26 set off with another naturalist, H. W. Bates, on a collecting tour in South America—hoping by the sale of specimens to cover the expenses of travel. Like Lyell and Darwin, he was an enthusiastic entomologist, and had conceived the same passion for travel. He had, as we have already seen, been deeply impressed by reading the Principles of Geology, and after spending four years in South America undertook a second collecting tour, which lasted twice that time, in the Malay Archipelago.
Before leaving England in 1848, Wallace had read and been impressed by reading the Vestiges of Creation, and there can be no doubt that from that period the question of evolution was always more or less distinctly present in his mind. While in Sarawak in the wet season, he tells us, 'I was quite alone with one Malay boy as cook, and during the evenings and wet days I had nothing to do but to look over my books and ponder over the problem which was rarely absent from my thoughts.' He goes on to say that by 'combining the ideas he had derived from his books that treated of the distribution of plants and animals with those he obtained from the great work of Lyell' he thought 'some valuable conclusions might be reached[112].' Thus originated the very remarkable paper, On the Law which has regulated the Introduction of New Species, the main conclusion of which was as follows: 'Every species has come into existence coincident both in space and time with a pre-existing closely allied species.' As Wallace has himself said, 'This clearly pointed to some kind of evolution ... but the how was still a secret.'