But, of still greater moment, is a letter in which Wallace tells Bates that he begins "to feel dissatisfied with a mere local collection. I should like to take some one family to study thoroughly, principally with a view to the theory of the origin of species." The two friends had often discussed schemes for going abroad to explore some virgin region, nor could their scanty means prevent the fulfilment of a scheme which has enriched both science and the literature of travel. The choice of country to explore was settled by Wallace's perusal of a little book entitled A Voyage up the River Amazons, including a Residence in Pará, by W. H. Edwards, an American tourist, published in Murray's Family Library, in 1847. In the autumn of that year Wallace proposed a joint expedition to the river Amazons for the purpose of exploring the Natural History of its banks; the plan being to make a collection of objects, dispose of the duplicates in London to pay expenses, and gather facts, as Mr. Wallace expressed it in one of his letters, "towards solving the problem of the origin of species." The choice was a happy one, for, except by the German zoologist Von Spix, and the botanist Von Martius in 1817-20, and subsequently by Count de Castelnau, no exploration of a region so rich and interesting to the biologist had been attempted. Early in 1848 Bates and Wallace met in London to study South American animals and plants in the principal collections, and afterward went to Chatsworth to gain information about orchids, which they proposed to collect in the moist tropical forests and send home. On 26th of April, 1848, they embarked at Liverpool in a barque of only 192 tons burden, one of the few ships then trading to Pará, to which seaport of the Amazons region a swift passage, "straight as an arrow," brought them on 28th of May.