Chapter 28
The origin and development of species and of living things; descent, variation and natural selection.
547 passages · 468 principal · Covers the 49 volumes of primary texts
The idea of evolution, as these books take it up, concerns the origin and development of living things. Are the kinds of plants and animals fixed from the beginning, or have they descended, with modification, from other kinds? If they have descended, by what means? It is a great idea for the Harvard Classics because one work in the collection, Darwin’s *Origin of Species*, made it the organizing question of natural history. Around that work stand Darwin’s own earlier *Voyage of the Beagle*, Lyell’s account of uniform change in the past, and scattered remarks by poets, essayists and philosophers. Some of these anticipate the notion of development in nature; others resist it. Darwin himself framed the question as a choice between two pictures: species as immutable creations, or species as modified descendants of forms now extinct1,2.
Darwin was careful to present his conclusion as the end of a long history of doubt. He traces it from Buffon and Lamarck, and finds even in Aristotle a foreshadowing of selection1. He credits Geoffroy and Wells with questioning the fixity of species, and notes that Wells recognized something like natural selection fitting human races to their climates3. Matthew too saw the principle4. The author of the *Vestiges* favoured development but relied on vague impulses that Darwin found scientifically inadequate to explain adaptation5. Owen’s statements on creation and origin he found ambiguous and confused6,7. By the time of Spencer, Wallace, Von Baer, Huxley and Hooker, many held that new species arise by a regular rather than a miraculous process, though Huxley still called the matter unproven8,9,10. Darwin’s own path ran through facts of distribution and geological succession, which first turned him toward “that mystery of mysteries”11,12. Yet he insisted that descent alone explains little until a means is shown that can account for coadaptation. External conditions, he argued, cannot make a woodpecker or a mistletoe13. He also explained why naturalists resisted the conclusion: we cannot see the long ages or the unseen steps through which slight variations accumulate14. And he criticized those who allowed variation in some forms while insisting that others were independently created15.
Outside strict argument, the notion of development appears in other forms. George Long finds in Marcus Aurelius’s chain of successive causes a “self-evolving power of nature”16. Emerson sees geology’s patient ages leading from granite to oyster to man and Plato17. He also notes rudiments of tiger and baboon in human form and races grading into one another over geologic time18,19. Tennyson writes that man’s making took millions of ages, that the monstrous eft once ruled the earth, and that man, now first, may not be last20. Lowell borrows the word itself for the drift toward popular government21. Other voices reject transformation. Harrison denies that dogs become wolves, because nature fixed a difference in form and bones22. Rousseau sets aside speculation about man’s animal beginnings23. He also dismisses the notion that ill-formed organisms were combined at random and the failures perished, asking why no failed forms are seen24. Pasteur leaves open whether two kinds of vibrio are adaptations of one stock, transformable by gradual change, or distinct species without common origin25. The ancients and early moderns therefore supply both the intuition of ascent and the objection that the record shows no failures. Darwin would have to answer both.
His answer begins with variation. Domestic organisms vary more than wild ones, because changed conditions of life induce and sustain variability26. Conditions sometimes act definitely, but more often they cause indefinite variation, and the nature of the organism outweighs the conditions themselves27,28,29. Use and disuse also modify inherited structures. Darwin cites flightless birds, wingless Madeira beetles and blind cave animals, though in each case natural selection works alongside disuse30,31,32. Taine, writing of literary history, likewise appeals to adaptation and inheritance over vast ages to explain the tenacity of primordial traits33. Darwin then sets out laws of variation. Changes in one part entail changes in others, by correlation34. Growth is balanced and economized35,36. Parts developed to an unusual degree remain highly variable37,38. Lost characters return by reversion, as in pigeons39. The stripes that appear on horses and asses he reads as reversion to a striped common ancestor of horse, ass, quagga and zebra40,41. Inheritance is the rule, and peculiarities tend to reappear in offspring at the age at which they arose in the parent42. Pigeon nestlings of different breeds accordingly differ far less than adults43.
From individual differences Darwin builds a continuous scale. Individual differences, varieties, subspecies and species blend by an insensible series44. Varieties are therefore “incipient species”45,46. Large genera yield more varieties, which separate creation gives no reason to expect47,48. Domestication supplies the analogy. Darwin argues that all pigeon breeds descend from the rock pigeon, citing reversion, the fertility of crosses and the improbability of many aboriginal stocks49,50. He notes that breeders doubt common descent because they attend only to differences and refuse to sum small variations51. Man’s accumulative selection, both methodical and unconscious, is the predominant power in forming domestic races52,53,54. The *Beagle*’s observations of Niata cattle and feral herds already show this interest in inheritance and drift of character55,56.
The mechanism Darwin proposes for nature is drawn from Malthus. Every organism tends to increase geometrically, so a struggle for existence is inevitable57,58. Food, enemies and climate check numbers, and climate acts chiefly by favouring competitors59,60. The *Beagle* had already explained extinction through such checks61. Organisms remote in the scale of nature are bound together, as in the chain of cats, mice, humblebees and clover62. Competition is fiercest among allied forms63,64. Natural selection is the preservation of favourable variations and the destruction of injurious ones: “survival of the fittest”65,66. It surpasses man’s selection, acting on every organ, over geological time, for the being’s own good67. Beside it Darwin places sexual selection, a struggle for mates. It is milder than natural selection because it does not entail death68,69.
Isolation aids this process, though Darwin denies that it is necessary70. Intercrossing keeps species uniform71. A large continent that oscillates in level, alternately broken into islands and reunited, most favours new and enduring species72. Divergence of character follows, because diversified descendants seize more places in the economy of nature73,74. Improved forms supplant their parents and the intermediates75. Extinction of the intermediates produces groups within groups76,77, and Darwin pictures the result as a tree of life with fallen branches78. In the *Beagle* he had already applied the same logic grimly to human varieties, the stronger extirpating the weaker79.
Does selection mean progress? Here Darwin parts from Lamarck’s innate tendency to perfection. Selection improves creatures relative to their conditions, and what counts as advance is obscure80. Lowly forms persist, and retrogression is possible81,82. Perfection is only relative, as endemic New Zealand forms yielding to introduced ones show83. Selection never acts solely for another species’ good84. Yet Darwin still speaks of new forms beating older ones and of organisation becoming higher85,86. This tension remains unresolved in the text.
Darwin set out the chief difficulties himself: missing transitional forms, organs like the eye, instinct, and hybrid sterility87. He meets them chiefly with gradations. Flying squirrels and Galeopithecus suggest steps toward flight88,89. Graded eye structures across related groups suggest how a perfect eye could be formed by slight modifications90,91. The theory would break down if any complex organ could not arise in this way92. Organs change function, as the swim bladder became a lung93. Electric organs in remote fishes arose separately, by analogous rather than inherited resemblance94,95. Against Mivart, Darwin explains the giraffe’s neck by the survival of the highest browsers in times of dearth96. He derives baleen from a lamellated, duck-like mouth97,98. He accounts for the flatfish eye by habit combined with selection99. Mimicry, he argues, begins with a rude accidental resemblance that selection then improves100,101. He concedes, too, that not every character is useful. Laws of growth and spontaneous variation account for some, though he denies that these can explain well-adapted structures102,103,104. Sudden origin he rejects on three grounds. Abrupt variations would be lost by intercrossing, allied species show fine gradations, and embryos differentiate by fine steps105,106.
Instinct posed a similar problem. Instincts vary and are inherited, and selection accumulates their slight profitable variations107,108. Darwin applies this to the cuckoo’s parasitism, the slave-making ant and the hive-bee’s economy of wax109,110,111. Sterile neuter insects were the special difficulty, since they cannot pass on what they acquire112. Darwin’s answer is that selection acts on the family, as breeders select stocks113. He presents the case as a refutation of Lamarck’s inherited habit114. Hybrid sterility he treats as incidental to differences in the reproductive systems, neither a special endowment nor a product of selection115,116,117. Some varieties prove partly sterile when crossed, so fertility gives no essential line between species and varieties118,119.
The geological record is where Darwin leans most on Lyell. Lyell assumes continual extinction and replacement of species, with gradual change recorded only rarely120. He holds that gaps in the strata do not prove sudden revolutions, because fossils are preserved only where strata accumulate121,122. He reads Tertiary faunas as approaching the living one by gradual replacement123,124. Darwin adopts this view of an imperfect record to explain the missing links125,126. He insists that whoever rejects the record’s imperfection must reject his theory127. He admits openly that he cannot explain the absence of fossil-rich strata below the Cambrian128,129. In time, species change at unequal rates, and lost forms never reappear130,131. Extinction is rarity carried a step further132,133. Extinct forms fall between living groups134, and the same types succeed one another in the same regions135. The *Beagle*’s Toxodon, blending orders now separate, and the giant quadrupeds of the Pampas had first raised these questions136,137. Geikie, writing later, simply assumes such a succession of life and extends the notion of evolution to the continents themselves138,139.
Distribution gave Darwin his starting point and some of his strongest arguments. Each species arose in a single region and migrated outward140. The identity of species on distant mountains is explained by migration during the Glacial period141,142. The Andes acted as an ancient barrier, and Darwin asks why creation should have occurred twice on either side of it143. Galapagos species resemble the American mainland rather than islands with similar conditions144,145. Neighbouring islands hold representative tortoises, mocking-thrushes and graded finches146,147. In the *Beagle* Darwin still marvels at the “creative force” behind these forms148. By the *Origin* he attributes them to colonisation, competition and modification149,150.
Finally, the structure of classification itself is read as genealogy. The natural system is a pedigree, and community of descent is the hidden bond naturalists have sought151,152. Adaptive resemblances conceal blood-relationship153. Homologous limbs and serial homologies are inexplicable on ordinary creation, but follow from slight modifications of a common archetype154,155. Embryos of distinct species resemble one another because variations appear late and are inherited at corresponding ages. The embryo is thus a picture of the progenitor156,157. Harvey had observed long before that each animal in its development passes through forms from ovum to worm to foetus, though without drawing any genealogical conclusion158. Rudimentary organs, which Darwin refuses to explain as made for symmetry, are useless relics of an ancestral state159,160. He admits that their complete obliteration is hard to account for161.
Darwin’s conclusion reaches beyond the great classes. By analogy he suggests that all organisms may descend from one primordial form162. He prefers secondary causes to separate creations163, and the tangled bank closes the book as the work of laws164. Several questions remain open within the texts themselves. Darwin does not settle whether selection yields real advance. He concedes the Cambrian gap. He could not apportion the shares of use and selection165. Nor, he admits, can we say why some species migrate or change while others stay unaltered166.
Introductory essay written by Claude Opus 5.5 from the outline and the notes on every passage below; quotations are checked against this edition.
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