It is obvious, however, that if we give up the principle in one case,--if we do not admit that the variations of the primeval dog were intentionally guided in order that the greyhound, for instance, that perfect image of symmetry and vigor, might be formed,--no shadow of reason can be assigned for the belief that variations similar in nature and the result of the same general laws which have been the groundwork through natural selection of the formation of the most perfectly adapted animals in the world, man included, were intentionally and specially guided. Darwin, therefore, was unable to follow the distinguished botanist, Prof. Asa Gray, in his belief that "variation has been led along certain beneficial lines," like a stream "along definite and useful lines of irrigation." Darwin's conclusion was that, if we assume that each particular variation was from the beginning of all time preordained, then that plasticity of organization which leads to many injurious deviations of structure, as well as the redundant power of reproduction which inevitably leads to a struggle for existence, and, as a consequence, to a natural selection or survival of the fittest, must appear to us superfluous laws of nature. V. 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