In a letter to Wallace dated 20th April, 1870, Darwin says, "There has never been passed on me, or, indeed, on any one, a higher eulogium than yours. I wish that I fully deserved it. Your modesty and candour are very far from new to me. I hope it is a satisfaction to you to reflect--and very few things in my life have been more satisfactory to me--that we have never felt any jealousy towards each other, though in one sense rivals. I believe I can say this of myself with truth, and I am absolutely sure it is true of you." * * * * * But on one question, and that round which discussion still rages, the friends were poles asunder. There had been correspondence between them as to the bearing of the theory of natural selection on man, and in April, 1869, Darwin wrote, "As you expected, I differ grievously from you, and I am very sorry for it. I can see no necessity for calling in an additional and proximate cause in regard to man." In the fifteenth chapter of his comprehensive book on Darwinism, Wallace admits the action of natural selection in man's physical structure. This structure classes him among the vertebrates; the mode of human suckling classes him among the mammals; his blood, his muscles, and his nerves, the structure of his heart with its veins and arteries, his lungs and his whole respiratory and circulatory systems, all closely correspond to those of other mammals, and are often almost identical with them. He possesses the same number of limbs, terminating in the same number of digits, as belong fundamentally to the mammals. His senses are identical with theirs, and his organs of sense are the same in number and occupy the same relative position. Every detail of structure which is common to the mammalia as a class is found also in man, while he differs from them only in such ways and degrees as the various species or groups of mammals differ from each other. He is, like them, begotten by sexual conjugation; like them, developed from a fertilized egg, and in his embryonic condition passes through stages recapitulating the variety of enormously remote ancestors of whom he is the perfected descendant. Full-grown, he appears as most nearly allied to the anthropoid or man-like apes; so much does his skeleton resemble theirs that, comparing him with the chimpanzee, we find, with very few exceptions, bone for bone, differing only in size, arrangement, and proportion.