The Descent of Man by Charles Darwin

First published: 1871

Type of work: Science

The Work

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; therefore, 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.

Writing as a biologist, not as a moral philosopher or as a theologian, Darwin does not try to consider the implications that his theory has for the various religious and philosophical explanations for the origin of the human race—be they that man was made from clay and woman from a rib, or that the human form sprang from God’s forehead. The single attribute that separates humanity from the rest of the animal world, Darwin thinks, is moral sense. Moral sense, the offspring of conscience, is the result of an evolutionary process. He asserts conscience came to the human race from a struggle between duty (sympathy and the social instincts) and desire (the urge for complete freedom). The belief in God also evolved, originating in dreams and developing through “spiritual agencies” into gods. It is this application, a logical outcome of Darwin’s theories, that horrified both conservative Christians and idealistic philosophers, for the theory completely eliminated the validity of revelation or of supramundane enlightenment.

Darwin concentrates on the rise of civilization from savagery. Natural selection and the struggle for survival advanced the intellectual powers so that the history of human institutions is the history of the evolution of the intelligence. As tribes grew stronger, the members learned to perceive the consequences of their actions, thereby developing moral sense. Then, as people became more and more aware of their moral sense, advanced civilizations with sophisticated religions and technologies were able to develop.

This discussion of the rise of civilization brings Darwin to the differences between races. Because individual members of different races can be mated so as to form fertile offspring and because the similarities between races far outnumber the differences, Darwin assumes that races are subsets of the one species. Furthermore, Darwin discards the hypotheses that each race descended from a primal pair, that the racial differences were caused by the conditions of life, and that the races evolved independently. The only theory that can explain the differences between races is sexual selection. The question of sexual selection, necessary to prove his assumption about race, leads Darwin into the argument that fills two-thirds of his book.

In On the Origin of Species, Darwin bases his theory of evolution primarily on natural selection or the struggle for survival. In other words, a slight modification in an animal’s structure might allow it to survive whereas another animal that lacked this modification would die. Existence, then, is a continual warfare in which the animal with the slight advantage wins. In The Descent of Man Darwin considerably modifies his view of nature by analyzing sexual selection, a different kind of biological warfare. When animals have their sexes separated, the male and female organs of reproduction differ; these are primary sexual characteristics. There are other differences, however, not directly connected with the act of reproduction, and these are secondary sexual characteristics. Usually the males have the most pronounced secondary sexual characteristics (for example, the brilliant plumage of many male birds); the males acquire these characteristics not from being better fitted for existence (natural selection) but from having gained advantage over other males and having transmitted their advantages to male offspring. There are usually more males than females, so that there is a struggle among males for the possession of the female; hence the female has the opportunity of selecting one out of several males. The strongest females have first choice among the males; therefore, the secondary sexual characteristics that pass through the strongest male and female have the most chance of outnumbering those characteristics that pass through weaker partners. In this way, the dominant characteristics are also the strongest. The more active the rivalry among the males, the more pronounced will be the variations between male and female.

Basing his analysis of the animal world upon these principles, Darwin begins with the lower classes of the animal kingdom. In the lowest classes these characteristics are absent because most often the sexes are joined in the same individual, but in the subkingdom of the Arthropoda undoubtable examples of secondary sexual characteristics appear. Darwin presents his most convincing case, however, in his long discussions on insects and birds. In both of these subkingdoms the characteristics are so clearly noticeable that Darwin accumulates material until he overwhelms his readers. Readers are presented with so much detailed information that they are willing to accept the evidence submitted, and the conclusions the evidence indicates, before Darwin applies his conclusions to humanity.

The secondary sexual characteristics of humanity are more complex than those of birds or of insects because humanity is more complex, but this is a difference only in degree. The adult male, for example, has a beard and hairiness of the body (although there is wide variation among tribes or races); he loves to fight and has greater endurance and strength than the female. As a result of his love of battle, the male delights in competition and develops his intellect more than woman, who is less selfish. Having noted a few of these characteristics, Darwin asks how they came to be. Because men vied with one another for the woman, the choice of the woman led to certain secondary sexual characteristics. For example, racial differences, he theorizes, are the result of ancient concepts of beauty; the remote ancestors of the African race preferred women who were dark-skinned and flat-nosed. Thus, Darwin is able to describe the differences among races without violating his basic theory of the descent of humanity from lower forms of life.

Darwin anxiously awaited the reaction to the publication of The Descent of Man and was surprised when he discovered that people were interested but not shocked. In fact, the book was anticlimactic. The disturbance caused by On the Origin of Species was calmed and was not stirred up again, and The Descent of Man became a book primarily for biologists.

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