"If we choose to let conjecture run wild, then animals, our fellow brethren in pain, disease, death, suffering and famine—our slaves in the most laborious works, our companions in our amusements—they may partake [of] our origin in one common ancestor—we may be all melted together."
"The different intellects of man and animals not so great as between living things without thought (plants), and living things with thought (animals)."
Speaking of intermediate forms, he remarks:—
"Opponents will say—show them me. I will answer yes, if you will show me every step between bulldog and greyhound."
Here we see that the argument from domestic animals was already present in his mind as bearing on the production of natural species, an argument which he afterwards used with such signal force in the Origin.
A comparison of the two editions of the Naturalists' Voyage is instructive, as giving some idea of the development of his views on evolution. It does not give us a true index of the mass of conjecture which was taking shape in his mind, but it shows us that he felt sure enough of the truth of his belief to allow a stronger tinge of evolution to appear in the second edition. He has mentioned in the Autobiography (p. 40), that it was not until he read Malthus that he got a clear view of the potency of natural selection. This was in 1838—a year after he finished the first edition (it was not published until 1839), and seven years before the second edition was issued (1845). Thus the turning-point in the formation of his theory took place between the writing of the two editions. Yet the difference between the two editions is not very marked; it is another proof of the author's caution and self-restraint in the treatment of his ideas. After reading the second edition of the Voyage we remember with a strong feeling of surprise how far advanced were his views when he wrote it.
These views are given in the manuscript volume of 1844, mentioned in the Autobiography. I give from my father's Pocket-book the entries referring to the preliminary sketch of this historic essay.
"1842, May 18,—Went to Maer. June 15—to Shrewsbury, and 18th to Capel Curig. During my stay at Maer and Shrewsbury ... wrote pencil sketch of species theory."[132]
In 1844, the pencil-sketch was enlarged to one of 230 folio pages, which is a wonderfully complete presentation of the arguments familiar to us in the Origin.