"It is impossible," says Marshall, "to depict better than this the condition prior to Darwin. In this essay there is full recognition of the fact of transition, and of its being due to natural influences or causes, acting now and at all times. Yet it remained comparatively unnoticed, because Spencer, like his contemporaries and predecessors, while advocating evolution, was unable to state explicitly what these causes were." Darwin and Wallace.--In 1858 we come to the crowning event in the rise of evolutionary thought, when Alfred Russel Wallace sent a communication to Mr. Darwin, begging him to look it over and give him his opinion of it. Darwin, who had been working upon his theory for more than twenty years, patiently gathering facts and testing the same by experiment, was greatly surprised to find that Mr. Wallace had independently hit upon the same principle of explaining the formation of species. In his generosity, he was at first disposed to withdraw from the field and publish the essay of Wallace without saying anything about his own work. He decided, however, to abide by the decision of two of his friends, to whom he had submitted the matter, and the result was that the paper of Wallace, accompanied by earlier communications of Darwin, were laid before the Linnæan Society of London. This was such an important event in the history of science that its consideration is extended by quoting the following letter: "London, June 30th, 1858. "My Dear Sir: The accompanying papers, which we have the honor of communicating to the Linnæan Society, and which all relate to the same subject; _viz_., the laws which affect the production of varieties, races, and species, contain the results of the investigations of two indefatigable naturalists, Mr. Charles Darwin and Mr. Alfred Wallace. "These gentlemen having, independently and unknown to one another, conceived the same very ingenious theory to account for the appearance and perpetuation of varieties and of specific forms on our planet, may both fairly claim the merit of being original thinkers in this important line of inquiry; but neither of them having published his views, though Mr. Darwin has for many years past been repeatedly urged by us to do so, and both authors having now unreservedly placed their papers in our hands, we think it would best promote the interests of science that a selection from them should be laid before the Linnæan Society. "Taken in the order of their dates, they consist of: