One of the disturbing features in this large negro population was the small proportion of women. Planters refused to invest in the latter, because they considered them unfit for the hard labor required. The result was such a surplus of male slaves that in some communities there were five hundred men to one negro woman. At first the negro slaves were employed mostly in the mines, where the native Indians had proved inefficient. Later they entered also domestic service. But with the development of agriculture, they began to be largely employed in the fields and on the plantations. Edward Gaylord Bourne says in his work on "Spain in America," the third volume in the historical series "The American Nation," in the chapter on Negro Slaves (p. 272): "The development of the sugar industry and the growth of slavery were dependent upon each other, especially after the mines of the Antilles gave out. Each trapiche, or sugar-mill, run by horses or mules, required thirty or forty negroes, and each water-mill eight at the least. Had the commerce of the islands been reasonably free, plantation slavery on a large scale would have rapidly developed, and the history of Hayti and the English islands would have been anticipated a century by the Spaniards." While Howard, Wilberforce, Judge Sewall and the Quakers are usually considered the pioneers of the abolition of slavery, the first voice raised against this institution came from Peru and was that of a Jesuit, Alfonso Sandoval, a native of Seville, but a resident of Peru, where his father held an important position in the royal administration. Sandoval wrote a work on negro slavery entitled "De Instauranda Aethiopum Salute," which was published in Madrid in 1647 and contains valuable data concerning the traffic, frequently quoted by historians. Nor can it be denied that the Spaniards knew better how to treat the negroes than either the French or the British. Evidences to the contrary suggest that whatever may have been the wrongs under which the negro slaves of the Spanish colonies suffered, they were not as much due to the cruelty of the masters, as to their ignorance and carelessness.