crustaceans, insects, fish, birds, reptiles, and amphibians. The focus of The Descent of Man is the primate order: in arguing for human kinship with other animal species, Darwin relies heavily on fossil evidence of archaic humans and secondary data about other primate species. In the first chapter, by repeating Huxley’s arguments, adding a few details, and placing the whole discussion within the context of speciation, Darwin gives his stamp of approval to the theory that humans, like other species, are simply the most recent stage in an evolutionary chain that—granting a few missing links—goes back to an ape-like ancestor.
However, whereas Huxley’s morphological argument ends with an assertion of bodily likeness between humans and other apes, Darwin’s explanation begins with morphology and quickly moves on to arguing similarities in human and animal cognition, psychology, society, and culture. In fact, Darwin distrusted Linnaean classification, and in The Descent of Man, he notes especially the case of the New World capuchin monkeys, the various forms of which some naturalists rank as species and others as varieties: “If of a cautious disposition, [the classifier] will end by uniting all the forms which graduate into each other as a single species; for he will say to himself that he has no right to give names to objects which he cannot define.”14 For Darwin, the study of life should concentrate not on external form but on processes within the lives of individuals and over evolutionary time. Darwin’s arguments about animals, including humans, are informed by attention to behavior and even attempts to understand motivation. He reasoned that human emotions and human society provide clues for understanding the lives of other animals, especially nonhuman primates, who can in turn serve as a mirror for a deeper understanding of human behavior.
In Darwin’s view, then, it matters less how many millet seeds it takes to fill up a brain case than how the brain—or any other organ—works. In The Descent of Man, Darwin begins with taxonomy and morphology, but most of his additions to Huxley’s work on nonhuman primates are physiological rather than morphological details. In his discussion of skin and hair, for instance, Darwin suggests that the relative scarcity of hair on human bodies evolved partly as a result of sexual selection and perhaps also because relative hairlessness helps humans remain free of parasites (a function of social grooming among other primates).
One of Darwin’s sources of information for The Descent of Man was a study of New World monkeys made by a German physician and explorer of Paraguay, Johann Rudolph Rengger, whose interests, like Darwin’s, lay in function and behavior over form. Against the trend of the time, and in spite of obstacles, Rengger made keen observations about the behavior of capuchin monkeys and speculated about their physiological processes. Citing Rengger, Darwin argues that close ties between humans and other primates are strongly suggested by these monkeys’ susceptibility to human diseases—bad colds, tuberculosis, “apoplexy, inflammation of the bowels, and cataract in the eye. The younger ones when shedding their milk-teeth often died from fever.”15 More interesting than these maladies is the monkeys’
strong taste for tea, coffee, and spirituous liquors: they will also, as I have myself seen, smoke tobacco with pleasure. Brehm asserts that the natives of north-eastern Africa catch the wild baboons by exposing vessels with strong beer, by which they are made drunk. He has seen some of these animals, which he kept in confinement, in this state; and he gives a laughable account of their behaviour and strange grimaces. On the following morning they were very cross and dismal; they held their aching heads with both hands and wore a most pitiable expression: when beer or wine was offered them, they turned away with disgust, but relished the juice of lemons. An American monkey, an Ateles [spider monkey], after getting drunk on brandy, would never touch it again, and thus was wiser than many men. These trifling facts prove how similar the nerves of taste must be in monkeys and man, and how similarly their whole nervous system is affected. 16
Today, diseases transmitted from humans are one of the leading causes of captive primate death, and even a casual visitor to research and shelter facilities is sometimes required to provide documentation of a tuberculosis test. Darwin was clearly in-trigued by Rengger’s research—and both men would perhaps be puzzled by how often biomedical and behavioral researchers continue to test the simian nervous system by giving monkeys cigarettes, beer, and other addictive substances.
Other likenesses Darwin notes between the human sensorium and that of other primates include similarities in nose, eyes, ears, and vocal apparatus. Humans have more prominent noses than most other primates, but it is impossible to distinguish humans from all other primate species on this basis, since the Hoolock gibbon has an aquiline nose and some monkeys, such as the proboscis monkey, carry the nose “to a ridiculous extreme.”17 All primates have similar facial structures; Darwin does not mention binocular or trichromatic vision (his resources for investigating neurology were limited), but he does note that many nonhuman primates have human-like eyebrows. Like some humans, certain monkeys also have vestigial Mr. Spock points on their ears, and though a distant ancestor common to all primates probably had moveable ears, this trait has not been preserved in apes: “The ears of the chimpanzee and orang are curiously like those of man,” Darwin writes, “and I am assured by the keepers in the Zoological Gardens that these animals never move or erect them; so that they are in an equally rudimentary condition, as far as function is concerned, as in man.”18 (Some humans can wiggle their ears, however; it’s a good party trick.) Darwin goes on to speculate that the strength and arboreal habits of the great apes and archaic humans might have protected them from the kinds of danger that would have made sharper hearing necessary for ground-dwelling species.
Of all primate physiological functions, Darwin seems most intrigued by the vocal prowess of some monkeys and apes, particularly the “singing gibbon,” whose long calls are more powerful than any human voice, even that of a trained opera singer. Gibbon calls, remarkably, travel farther even than the long calls of gorillas and orangutans. One gibbon, the Hylobates agiles, Darwin notes, can reproduce an entire octave of musical notes, “which we may reasonably suspect serves as a sexual charm,” just as music functions in the human species.19 (Curiously, W. C. L. Martin, one of Darwin’s sources, reproduces the call in musical notation.) Many simians, Darwin remarks, are musical and convey emotions by tonality, as Poe anticipates when his witnesses to the murders in the Rue Morgue comment on the orangutan’s supersyllabics and tone. Darwin thus believes that most primate calls function as communications of important information about the individual’s surroundings or emotional state, or they serve as a kind of vocal ornamentation in reproductive competition among males. But among the New World species studied by Rengger, both sexes of some howlers evidently sing because “they delight in their own music and try to excel each other.”20 In this way, Darwin speculates, monkey calls are probably similar to the music and poetry of archaic humans. Ever intrigued by sex, he proposes that in humans singing evolved before speaking—because it added sex appeal and could express emotion as well as information.
Darwin also devotes a great deal of space to the limbs and extremities, but unlike Huxley, he explains function along with anatomy. Darwin’s investigations lead him to believe that evolutionary pressures among the various primate species have resulted in hands specialized for climbing (monkeys and apes) or refined manual dexterity (humans), but it is physically impossible, he theorizes, for a hand to be perfectly suited for both. Likewise, Darwin considers legs, feet, and toes as they have adapted to locomotion, concluding that bipedalism, an adaptation to open spaces, would be maladaptive in arboreal environments, where food, shelter, and safety depend on ease of movement in trees. He suggests that human bipedalism initiated a cascade of other adaptations—feet that support weight, a spine curved for balance, a broadened pelvis. Bipedalism also freed the hands, making powerful jaws and large teeth unnecessary as weapons and even less important for chewing, since food could be manually modified.
But Darwin stops short of relating increased intelligence and capacity for speech to bipedalism, as some more recent evolutionary biologists have suggested. In his view, human mental powers originated in primate social life in all its dimensions. Indeed, almost everything Darwin has to say about primate morphology, bodily functions, and senses is connected to speculations about behaviors shared by humans and their cousins, the apes and monkeys.
Like most twentieth-century primatologists, Darwin