The Descent of Man | Study Guide

Charles Darwin

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The Descent of Man | Chapter 2 : On the Manner of Development of Man from Some Lower Form | Summary

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Summary

Darwin begins Chapter 2 by stating that humans are subject to great variability and diversity. No two people are precisely alike in facial features, body, arms and legs, internal organs, muscles, teeth, or arteries. Nor are they alike in mental capacity, which is true among all animals: no two are exactly the same. Admitting that he and other naturalists remain fairly ignorant of what causes such variability, he asserts that there are some influences that warrant examination for both human and animal. He compares humans to domesticated animals: both encounter far more variable conditions than wild animals, whose environments are much more predictable. Thus humans are more likely to need to adapt to change. One important factor that distinguishes humans from animals in this regard is that humans have far more variation in procreation, unlike farm animals whose breeding is often highly controlled by their owners.

Darwin suggests there are nine "laws of variation" in humans' evolution, although he chooses to focus here on the most important ones. A changed condition appears to produce an effect on organisms, though he admits that this is difficult to prove precisely. Darwin cites an analysis of one million Civil War soldiers as evidence of the difficulty of clearly proving this effect of condition, or environment. It was found that the soldiers from Western states—where populations were low and space is vast—were generally taller than all the other soldiers. One million subjects is an impressive sample. But a French scientist did a similar study of French soldiers and found nearly the opposite result: cold weather will increase hair growth in mammals but not in humans—who share many similar physical structures with animals. Darwin concludes that while climate must certainly have some effect on mammals' internal organs (lungs, kidneys) and skin, proving the effects remains difficult.

Increased use and disuse of body parts play a key role in variability as well as heredity, according to Darwin. If a man frequently runs, his leg muscles get stronger; if he doesn't run much, the muscles will weaken over time. The same effect applies to eyesight, bones, the heart, teeth, etc. If one kidney fails, the remaining kidney will grow in size from use. The leg bones of a large man who works on his feet all day will become thicker over time. Darwin believes these variations in humans eventually get passed along to generations through heredity. The Payaguá of South America had thin legs and thick arms as a result of generations living off the river and spending so much time traveling in canoes.

Darwin asserts that in earlier epochs, progenitors (original ancestors) of man were in a transitional state, changing from quadrupeds (walking on all fours) to bipeds (walking on two feet). In both apes and humans, newborns have thick skin on the soles of their feet and soft skin on their palms—a fact Darwin points to as proof that thousands of years of adapting to conditions (walking on two feet specifically) created a hereditary feature of the species. He also provides the example of the South American Aymara, who live at a cold 10,000–15,000 feet altitude and whose legs, over time, have developed uniquely: their thighs are notably shorter than their shinbones. When animals are moved from their familiar environments, adaptability can take generations. Darwin points out that the Aymara, when moved south to a lower, warmer climate, suffered a very high death rate initially. Those who survived for two generations were able to adapt (natural selection), and while their unique legs remained, more and more of their legs began to decrease in shinbone length over time.

Darwin uses the example of microcephaly (humans with small skulls and often related mental limitations) to discuss arrests (interruptions or premature cessations) of development. These humans' brains didn't develop normally, and they often share physical and functional characteristics with lower animals such as apes: they both have pronounced brows and projecting jaws, are physically very active (sometimes moving around on all fours), are unable to speak, smell their food, have very short attention spans, and are of limited intelligence. Reversion—a return to a previous state—is connected to arrested development. In reversion a long-lost or unused structure is called back into existence. Darwin suggests that lower members of groups can give us an idea of how the common progenitor was constructed. He gives many examples of this, including a discussion of how marsupials (e.g., kangaroos, possums), a lower form of mammal related to humans, have two uteruses yet higher forms of animals, such as humans and apes, have a single uterus. Darwin asserts that because marsupials and humans descend from the same early progenitor, both must have had two uteruses at one point, and natural selection, adaptation, and heredity caused humans' uteruses to coalesce into one.

Rate of increase in population is another key factor, according to Darwin, in the development and variability of animals. Where food and safe shelter are readily available, populations tend to increase. Darwin points out that in the United States the human population doubled in only 25 years. Civilized societies tend to have more children because of the ability to sustain more children's lives; savage tribes tend to have fewer children because of the harsher reality of providing food and safe shelter. Other factors—disease, infant mortality, and wars—also play a role in controlling the rate of population increase. Darwin cites the Santal, a hill tribe in India, whose population increased by its avoidance of wars and the introduction of vaccines, despite the fact that marriages were delayed until later in life.

The last half of the chapter is dedicated to Darwin's further explanation of his theory of natural selection, employing the research he'd done (as well as that of other scientists) since his On the Origins of Species was published. He begins by pointing out that beneficial variations tend to be preserved and passed along through heredity; injurious variations are eliminated over time. Human intellect is the key to their dominance over the earth and other animals, according to Darwin. Higher animals can create basic tools of some kind—apes regularly use sticks and rocks—but humans' intellect allows them to create and refine tools to a degree unmatched by any other animal. And while many animals can on occasion use division of labor to accomplish a greater feat than one animal alone, Darwin suggests early man-made division of labor a central part of its community. Humans' advanced language skills and social structures are clearly beyond even the highest animals in Darwin's assessment.

As early humans stopped walking on all fours and became upright, many physical changes occurred: legs lengthened, hips widened, the turning range of the neck became greater, and humans' hands became more agile. Intellectual changes occurred as well: the brain grew larger, language developed, and innovation advanced man's creation of tools and organizational structures for communities. Darwin provides many specific examples of the natural selection changes between early progenitor man and its descendants and its related animal cousins:

  • Hair: Man grew to be relatively hairless; monkeys and apes remain hairy.
  • Teeth: Man's canine teeth became less prominent from disuse in eating meat; monkeys and apes retain more prominent canines.
  • Tails: Protoman's tail disappeared, probably because of its interference when walking, sitting, etc.; many monkeys still have tails, as do other related mammals.
Darwin admits that he may have put too much emphasis on natural selection (or "survival of the fittest," as some misinterpreted his earlier work) as the primary cause of human development. The abundant research done by his supporters and detractors, along with his own, forced him to change new editions of On the Origins of Species to confine the theory to "adaptive changes of structures." But he feels that natural selection is still valid and has been helpful in overthrowing the idea that humans and animals were created separately.

Analysis

By providing a wealth of supportive evidence, Darwin intends to advance his ideas about humankind's descent from a lower animal and development through natural selection. He focuses on a few important elements—variability, adaptation, and structural changes—to help explain to readers how a progenitor lower animal could possibly develop into humans and other related mammals (apes, monkeys, dogs, horses, etc.). However, due to the fact that the genetic basis for these changes was unknown by Darwin at the time, he occasionally conflates what we know today are phenotypic (the physical characteristics of an individual) changes that can occur over the life span of an individual and that may not necessarily be passed down from generation to generation with genetic change. For instance, Darwin discusses the changes in body parts due to increased or decreased use. These individual-level variations, due to different degrees of use, are phenotypic changes that occur at the level of the individual rather than genetic changes at the level of a population. Darwin focuses on these types of changes because, based on his research and the science he encountered from other naturalists, they make the most sense as primary causes of humankind's physical and mental development.

He often compares civilized man to savages of the 19th century as a way to show how environments (condition is his preferred term) impact human development and can be seen and documented even in contemporary times. Because his point is that variability has played such an important part in the human animal's development, he shows how the same mammals sharing nearly exact physical structures but existing in two different conditions reflect the important impact of environment. Darwin also makes similar comparisons between humans and monkeys to show how closely related mammals may have diverged in their development from the same progenitor lower animal and become two distinct animals while remaining close cousins.

He provides some social context for his analysis as well, suggesting that ideas and laws of community played a part in the descent of man. (He will explore this topic further in the next two chapters.)

The effect of providing so much scientific evidence (his own and other scientists') to support his assertions is that the reader gets both a broad understanding of Darwin's theory of natural selection and data-based evidence that relates to the reader as a human animal. Though he admits he may have gone a bit far in ascribing too much of man's development to natural selection alone, he is no way abandoning his theory—in fact, this chapter and the entirety of The Descent of Man are intended to bolster his theory.

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