shown by their papers, Darwin and Wallace differed considerably in the attitude from which they approached and regarded the problem,_As Dr. Osborn points out, "Darwin dwells upon variations in single characters, as taken hold of by Selection; Wallace mentions variations, but dwells upon full-formed varieties, as favorably or unfavorably adapted. It is perfectly clear that with Darwin the struggle is so intense that the chance of survival of each individual turns upon a single and even slight variation. With Wallace, Varieties are already presupposed by causes which he does not discuss, a change in the environment occurs, and those varieties which happen to be adapted to it survive." Needless to say, of the two methods of approach, that of Darwin is by far the more thorough-going.
Wallace resigned the work in favor of Darwin, who in 1858 set to work upon a book on the development of species, but was so interrupted by illness that more than a year was necessary to complete it. In the main it was a condensation of the work begun in 1856 and interrupted by the arrival of Wallace's paper—a condensation which Darwin did not desire, but undertook in order to prevent further difficulty in matters of priority, and to gratify his friends. Throughout the period of the work there are references to it in his letters—here the refusal to read a "heavy" book until the work on the "Abstract" is done—there a wish that the work would be completed, so he could rest. On January 25th, 1859, Darwin wrote Wallace, "… thank God I am in my last chapter but one," while on March 2d he asked Hooker
