Next to the “Origin of Species,” the volume which sets forth Darwin’s theory of the “Descent of Man” naturally excited the most widespread attention.  This book, which took the author three years to write, was published in 1871, a second and carefully revised edition appearing three years later.  The data brought together occupy more than six hundred pages.  The conclusions reached may be summed up in a few paragraphs.  The principal induction from the evidence is that man is descended from some less highly organized form.  It was Darwin’s conviction that the grounds upon which this conclusion rests will never be shaken, for the close similarity between man and the lower animals in embryonic development, as well as in innumerable points of structure and constitution, both of high and of the most trifling importance,—­the rudiments which he retains and the abnormal reversions to which he is occasionally liable,—­are facts which cannot be disputed.  Viewed in the light of our knowledge of the whole organic world, their meaning is unmistakable.  The great principle of evolution stands out clear and firm when these groups of facts are considered in connection with others, such as the mutual affinities of the members of the same group, their geographical distribution in past and present times, and their geological succession.  It is pronounced incredible that all these facts should speak falsely.  He who is not content to look like a savage at the phenomena of nature as disconnected cannot any longer believe that man is the product of a separate act of creation.  He will be forced to admit that the close resemblance of the embryo of man to that, for instance, of a dog,—­the construction of his skull, limbs, and whole frame on the same plan with that of other mammals, independently of the uses to which the parts may be put; the occasional reappearance of various structures, for instance, of several muscles which man does not normally possess, but which are common to the Quadrumana, and a crowd of analogous facts,—­all point in the plainest manner to the conclusion that man is the co-descendant with other mammals of a common progenitor.

Darwin recognized that the high standard of our intellectual powers and moral disposition constitutes the greatest difficulty which presents itself after we have been driven by the mass of biological evidence to accept his conclusion as to the origin of man.  Touching this point, he observes:  “Every one who admits the principle of evolution must see that the mental powers of the higher animals, which are the same in kind with those of man, though so different in degree, are capable of advancement.  Thus the interval between the mental powers of one of the higher apes and of a fish, or between those of an ant and scale-insect, is immense; yet their development does not offer any special difficulty, for with our domesticated animals the mental faculties are certainly variable, and the variations are inherited.  No one doubts