Darwin spells out in his introduction his three-fold purpose. He will first consider whether man, like all other species, is descended from an earlier form. Then he will discuss the way man has developed. Finally, he will evaluate the differences among the human races.
Darwin develops his arguments through many examples from a wide range of sources. The first two chapters describe physical correspondences between man and the other vertebrates, concluding that to deny a common descent would be unreasonable. Only prejudice and arrogance, he says, would explain a rejection of this evidence.
Man’s mental powers are compared at length with those of the other animals. Darwin finds many animals capable of some reasoning, and he also finds parallels between man’s use of language and the cries of animals. Most significant, perhaps, are his tentative remarks about man’s religious instincts and the instincts of animals.
Darwin devotes eleven chapters to the principles of sexual selection among animals and to their secondary sexual characteristics. The four chapters on birds give much information on the sexual functions of their plumage, their calls, and their behavior patterns.
The last three chapters are a special section on “Sexual Selection in Relation to Man.” Much of the discussion in this part treats the role of beauty in determining marriages. Darwin also studies marriage customs among primitive people.
The firestorm of controversy that followed the publication of On the Origin of Species by Means of Natural Selection (1859) had died down considerably by the time Darwin decided to publish his book about the origin of the human species. Consistent with his meticulously detailed analysis of the origin of species within the animal kingdom, Darwin explains in The Descent of Man how the human animal, too, evolved from lower forms. The impact of his pronouncement was somewhat blunted on his contemporaries because many assumed these conclusions after reading the On the Origin of the Species; the famous argument between Thomas Huxley and Bishop Wilberforce, in which the latter vehemently denied that his family tree included any apes or monkeys, took place a decade before Darwin dared to publish his findings about human genealogy. Nevertheless, until Darwin spoke, lesser luminaries could be dismissed; once the great biologist made it clear that he held no privileged place for humankind in the evolutionary process, the rift between scientific and religious explanations for the creation was complete.
The chief scientific significance of Darwin’s work lies in his insistence on the prominence of sexual selection in determining human evolution. Important for the history of ideas, however, is Darwin’s insistence that no special privilege should be accorded to humanity’s “moral sense.” He insists that the development of moral qualities in human beings is simply a part of the normal process of evolution. In his system, there is no need for a God who creates the human soul or speaks directly to humankind to explain how to live. In the twentieth and twenty-first centuries, the implications of The Descent of Man proved to be more troubling for theologians than for scientists.
Problems beset Darwin when he turned from his brilliant biological study of 1859 to the more particular analysis of the relation of humanity to the natural world. These problems are immediately seen in the organization of Darwin’s argument: More than two-thirds of the book is an exhaustive discussion of sexual selection. The book may lack the inspiration of On the Origin of Species, but in its summary and evaluation of the anthropological thought after the publication of that earlier masterpiece, The Descent of Man is one of the most important books of the nineteenth century.
In his introduction Darwin says that he plans to consider three things: whether people descended from some preexisting form, how they developed if indeed they did so descend, and what value the differences between races have to such a development. He draws evidence of the descent of the human species from his vast knowledge of medicine and biology. That people share bodily structure, embryonic development, and rudimentary organs with other mammals seems to him to be evidence enough for asserting a common ancestry. Since anthropologists and paleontologists had not at that time discovered significant relics of prehistoric human life, Darwin’s affirmation of the descent of humanity is based on logic; thus he amasses an almost overwhelming number of analogies to strengthen his case. These analogies enable him to trace the development of humanity from lower animals, but in order to do so he must assume a definition of humanity. Darwin maintains that humanity’s uniqueness is not due to any one characteristic but to a combination of many: upright position, acquisition of language and tools, a delicate and free hand, and superior mental powers. In the possession of these traits humanity is different only in degree. In fact, Darwin musters evidence to show that animals have curiosity, imagination, attention, and reason, attributes that earlier philosophers thought set people apart from the rest of the animal world.