On the Origin of Species By Means of Natural Selection (1859) Informative Summary


Charles Darwin’s On the Origin of Species is a landmark work in the history of science, presenting a revolutionary theory of evolution by natural selection. Darwin, drawing upon his extensive observations during his voyage on the HMS Beagle, meticulously lays out his arguments, starting with the variability of domesticated plants and animals. He demonstrates how humans selectively breed for desired traits, leading to dramatic changes in species over generations. This observation leads him to propose that a similar process, “natural selection,” operates in nature.

Darwin posits that all organisms have the potential to reproduce at a geometric rate, but limited resources and competition create a “struggle for existence.” In this struggle, individuals with even slight advantageous variations are more likely to survive and reproduce, passing on those beneficial traits to their offspring. Over time, this accumulation of small variations leads to the gradual transformation of species, ultimately giving rise to new and distinct species. Darwin meticulously explores the complexities of natural selection, addressing potential objections and providing evidence from various fields like geography, embryology, and paleontology to support his theory.

Key Findings:

  • Species are not immutable but change over time. Darwin argues convincingly against the prevailing belief in the fixity of species, proposing that they evolve through a process of gradual change.
  • Natural selection is the primary driver of evolution. Darwin’s central thesis is that natural selection, the process by which organisms with beneficial traits are more likely to survive and reproduce, is the driving force behind evolutionary change.
  • The struggle for existence is fundamental to evolution. Darwin demonstrates that all organisms compete for limited resources, leading to a “survival of the fittest” where only those best adapted to their environment survive and reproduce.
  • Extinction is an inevitable consequence of evolution. As new, more adapted species emerge, older, less fit species are likely to be outcompeted and eventually become extinct.
  • The fossil record is a testament to evolutionary change. Darwin argues that the fossil record, though imperfect, provides evidence for the gradual transformation of life over vast geological periods.


  1. Domesticated animals and plants are more variable than their wild counterparts. This is because they are raised in environments less uniform than those found in nature and often receive more food. (7)
  2. Confinement affects the reproductive systems of animals. Many animals fail to breed freely under captivity, highlighting the sensitivity of their reproductive systems to changes in their environment. (8)
  3. “Sports” are extremely rare under nature but more common under cultivation. These sudden, dramatic changes in a single bud or offset demonstrate the influence of the parent’s environment on variability. (9)
  4. Habit has a decided influence on the variation of animals. For instance, the domestic duck, flying less and walking more than its wild ancestor, shows changes in bone structure. (11)
  5. Long limbs are often accompanied by an elongated head in animals. This correlation of growth demonstrates how changes in one part of the body can affect other parts. (11)
  6. The most flourishing or dominant species often produce well-marked varieties. These varieties, having inherited the advantages that led to their parent’s dominance, are likely to survive in competition. (52)
  7. Species of larger genera in a country tend to vary more than those in smaller genera. This reflects the higher rate of speciation occurring in larger genera, where there has been more variation and differentiation in the past. (55)
  8. Species very closely related to other species often have much restricted ranges. This resemblance to varieties suggests that they are likely to be recently evolved forms, not yet having had time to spread widely. (59)
  9. The struggle for existence is a universal principle. All organisms, from the smallest parasite to the largest mammal, compete for resources and space. (60)
  10. Every species has the potential to increase at a geometric rate. If unchecked, any species would quickly overpopulate the earth. (64)
  11. The number of eggs or seeds produced by a species is not a primary indicator of its abundance. A large number of offspring merely compensates for high mortality rates at an early stage of life. (66)
  12. Climate is a major factor in determining the average numbers of a species. It indirectly affects populations by reducing food availability and favoring the spread of competitors. (68)
  13. Introduced species often outcompete native species. This demonstrates that native species are not necessarily perfectly adapted to their environment and could be improved through natural selection. (71)
  14. The relationship between organisms is a key factor in determining their distribution. A change in one species can ripple through an entire ecosystem, affecting the numbers and distribution of others. (71)
  15. The struggle for existence is most severe between individuals of the same species. This competition for the same resources often leads to the displacement of less-adapted varieties. (75)
  16. Natural selection acts on variations, no matter how slight. Even seemingly insignificant traits can be crucial in the struggle for survival, leading to the gradual accumulation of advantageous features. (84)
  17. Natural selection can act on organisms at any stage of life. Modifications that benefit the young or larva, for example, will affect the adult form through correlation of growth. (86)
  18. Sexual selection is a secondary form of selection that focuses on traits related to mate acquisition. This process can lead to the development of exaggerated features in males, like horns or bright plumage, that enhance their chances of attracting females. (87)
  19. An occasional intercross with a distinct individual is likely a universal law of nature. This process, demonstrated in both plants and animals, appears to be essential for maintaining vigor and fertility. (96)
  20. Isolation is an important factor in the process of natural selection. By limiting interbreeding and the influx of competing species, isolated areas allow for the development of new and distinct forms. (104)


  1. 182 British plants are generally considered as varieties but have been ranked as species by botanists. (48)
  2. 251 British plants are listed as species by Babington, while Bentham lists only 112, showcasing the controversy surrounding species vs. varieties. (48)
  3. 139 doubtful forms of British plants exist, reflecting the difficulty in classifying species and varieties. (48)
  4. 182 British plants are generally considered as varieties, but have all been ranked as species by botanists. (48)
  5. 63 British plants considered as doubtful species by Mr. H. C. Watson, on average, range over 6.9 of the provinces of Great Britain. (59)
  6. 53 British plants considered as varieties, on average, range over 7.7 provinces. (59)
  7. 200 out of 550 species of beetles on Madeira are wingless and cannot fly. (135)
  8. 23 out of 29 endemic genera of beetles on Madeira have all their species wingless. (135)
  9. 295 out of 357 seedlings of native weeds were destroyed, mainly by slugs and insects. (67)
  10. 9 out of 20 species of plants growing on a small plot of turf perished when allowed to compete freely. (67)
  11. 12 species of plants flourished in a Scotch fir plantation on a barren heath, compared to none on the untouched heath. (72)
  12. 32 young Scotch fir trees were counted in one square yard of enclosed heath, where cattle could not graze. (72)
  13. 260 naturalized plants are enumerated in the flora of the Northern United States, belonging to 162 genera. (115)
  14. 100 out of 162 genera of naturalized plants in the Northern United States are not indigenous. (115)
  15. 2048 is the proportion of an ancestor’s blood in a breed after 12 generations. (160)
  16. 306,662,400 years is the estimated time required for the denudation of the Weald, assuming a cliff 500 feet in height to erode at a rate of 1 inch per century. (286)
  17. 600 feet is the estimated rate of sediment deposition by the Mississippi river in 100,000 years. (284)
  18. 1400 feet thick is the carboniferous bed in Nova Scotia, containing root-bearing strata at 68 different levels. (296)
  19. 46 flowering plants are common to Tierra del Fuego and Europe, and still exist in North America. (374)
  20. 25 species of algae are common to New Zealand and Europe but not found in the intervening tropical seas. (376)


  1. Natural Selection: The process by which organisms with advantageous traits are more likely to survive and reproduce, leading to the gradual evolution of species. (80)
  2. Species: A group of organisms that can interbreed and produce fertile offspring. (44)
  3. Variety: A form within a species that differs from the typical form but can still interbreed with it. (44)
  4. Hybrid: The offspring of two different species. (245)
  5. Mongrel: The offspring of two different varieties of the same species. (245)
  6. Polymorphic: A species exhibiting a high degree of variation within its population. (46)
  7. Incipient Species: A variety that is in the process of becoming a distinct species through natural selection. (52)
  8. Analogical or Adaptive Characters: Features that are similar in appearance and function but have evolved independently in different lineages. (427)
  9. Homologous Organs: Organs in different species that have a similar underlying structure, reflecting their shared ancestry, even if they have evolved to serve different functions. (434)
  10. Rudimentary Organs: Organs that are reduced in size and function, often reflecting the loss of a function that was important in the ancestor. (450)


  1. Woodpecker and Misseltoe: Darwin uses these organisms to illustrate the complexity of adaptations, showcasing how the woodpecker’s beak and feet are perfectly adapted for catching insects under the bark of trees, while the misseltoe, a parasitic plant, depends on specific trees for nourishment and birds for seed dispersal. (3)
  2. Domestic Duck: The bones of the wing of the domestic duck weigh less and the bones of the leg more than in the wild duck. Darwin attributes this to the domestic duck flying less and walking more. (11)
  3. Cats: Cats with blue eyes are invariably deaf, while those with tortoise-shell coloring are invariably female. These whimsical examples demonstrate the existence of correlation of growth. (12)
  4. English Carrier Pigeon: The beak of the English Carrier pigeon is abnormally long compared to other pigeons, even those within the same family. Darwin argues this extreme variation was likely selected for by breeders. (22)
  5. Fantail Pigeon: The Fantail pigeon has an abnormally large number of tail feathers, demonstrating how breeders can select for extreme, even somewhat monstrous, traits. (22)
  6. Scotch Fir: Darwin observes that cattle can completely determine the presence or absence of Scotch fir trees in an area, highlighting the complex and indirect relationships between species. (71)
  7. Humble-bees and Clover: Humble-bees are essential for the fertilization of red clover, as other bees cannot reach the nectar. Darwin hypothesizes that if humble-bees became extinct, red clover would likely disappear as well. (73)
  8. Flying Squirrels: Darwin examines the gradual transition from squirrels with slightly flattened tails to those with fully developed flank membranes, which allow them to glide, demonstrating the potential for the slow accumulation of advantageous traits. (180)
  9. Galeopithecus: This gliding mammal, once mistakenly classified as a bat, showcases how intermediate forms might have existed in the past, connecting it to other lemurs. Darwin proposes that further elongation of its membrane-connected fingers could have led to the development of flight. (181)
  10. Water-ouzel: This diving bird, a member of the terrestrial thrush family, illustrates how habits can change without a corresponding change in structure, suggesting that the bird’s aquatic adaptations evolved more recently than its overall thrush-like structure. (185)

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