The Descent of Man | Study Guide

Charles Darwin

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The Descent of Man | Chapter 6 : On the Affinities and Genealogy of Man | Summary

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Summary

Darwin opens the chapter asserting the essential claim of his theory of evolution: "man is descended from some lower form," even though definitive connecting links have not yet been discovered. Humans and other mammals share the same progenitor, and they share many similarities in embryonic development as well. He takes issue with naturalists who put humans in their own kingdom. He compares this to ants and scale insects: the difference between the smarter ants and the scale insects is immense, but ants aren't put into a different class or kingdom. So it is with humankind and the higher animals: they have similar structures, features, muscle movements, expressions—human intelligence is much higher, but that doesn't mean man should be in his own kingdom.

Darwin agrees with the naturalists that classify humans as descendants of the Quadrumana (primates with four hands), and he catalogues many of the features and structures that humans and monkeys (also descendants of the Quadrumana) share. He points out that humans and monkeys are classified as Old World monkeys (as opposed to apes, which are New World monkeys), though both types are descended from the same lower form. He says it's best to consider humans as "but one of several exceptional forms of primates." Darwin suggests that it is probable that humans' early progenitor lived on the African continent, though it is unknown how long ago man diverged through natural selection from the Old World monkey stock. It may seem "monstrous" to those that do not understand natural history, says Darwin, but the fact is all vertebrate animals (possessing a spine or backbone) are descended from the same singular progenitor, which means humans, reptiles, birds, and fish all began from the same original source.

Darwin describes the features and structures of the early progenitor of man:

  • hair-covered
  • pointed ears that could move
  • tails
  • arms and legs like the Quadrumana
  • prehensile (grasping) feet
  • tree dwellers preferring warm forests
  • large canine teeth
  • third eyelid to protect eyes
  • lungs like swim bladders (early progenitors were probably aquatic)

For progenitor mammals, females had two uteruses, and males probably had rudimentary uteruses and mammary glands. Darwin points out that modern males still have mammary glands, though not fully developed or functional. He suggests that both sexes nourished infants at one point, but as males ceased to do so, disuse caused an adaptation that continued to be passed down genetically through generations.

Darwin closes the chapter with an account of how mammals evolved from their early progenitor, ancient monotremes (duck-billed platypus, echidnas), to ancient marsupials (e.g., kangaroos, possums, koalas) to Lemuridae (lemurs) to Simiadae (apes, monkeys). These split into New World monkeys and Old World monkeys, and from Old World monkeys descended man. Though this may be a simplistic account, Darwin makes the point that if any of the evolutions in the chain didn't exist, man would not be exactly who he is.

Analysis

Darwin explores the genealogical development of man from his progenitor lower form in order to argue against contemporary naturalists that believe man should be classified as its own kingdom of animal. Darwin may have believed this to be sheer ego on the part of some naturalists because the only reason this is suggested is because man is deciding on the classifications. His evidence for the similarities between man and monkey—as well as other mammals—argues for keeping them all in the same class.

The descriptions he provides of what man's progenitor may have looked like cover many of the details he has previously explained in earlier chapters, though listing them as he does here does evoke an image for the reader. It's easy to "see" many of the common features of the progenitor in animals that readers would be familiar with—humans, monkeys, dogs, horses, etc. Though he says it's probable that early man lived in Africa, he doesn't provide solid evidence for this. He instead relies on a logical connection between man's progenitor's physical structure and the kind of environment that could support him and lead to adaptation (and thus natural selection).

The progression Darwin suggests for the genealogy of man and his evolution from ancient monotreme (duck-billed platypus) to mankind is simplistic, though he makes no claim that it is comprehensive. His purpose seems to be to give the reader a general view of how evolution and natural selection may have progressed based on the evidence he's presented in the book so far.

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