Darwin's 'One Long Argument' in the Origin

Lecture notes, ca. 1987.

Parenthetical page references are to On the Origin of Species by Charles Darwin: A Facsimile of the First Edition, edited by Ernst Mayr. Harvard University Press, 1964.

Darwin refers to the Origin as "one long argument" (p. 459). What is the "argument"? We have seen Darwin argue along these lines:

1. Individuals within a species vary. (See e.g. p. 45: "no one supposes that all the individuals are cast in the very same mould. These individual differences are highly important for us, as they afford the materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his domesticated productions.")

2. Much individual variation is heritable. (E.g. p. 12, "any variation which is not inherited is unimportant for us"; p. 13, "perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the exception.")

3. Organisms multiply at a rate which exceeds the capacity of their environment to support them, and so many die. (E.g. pp. 63, "A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase.... As more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.")

It follows from (1)-(3) that

4. Organisms that are better suited to their environments -- "fitter," though Darwin eschews this expression -- will survive and reproduce at a higher rate than those which are less well suited to their environments. (E.g. p. 61, "Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring.")

The fact captured by (4) is the fact that Darwin calls "descent with modification," and it is the key to understanding the origin of species. So Darwin writes in the Introduction to the Origin:

I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. (6)

The view is expressed in the concluding sentence as well:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. (490)

In a letter to his friend and critic Charles Lyell, Darwin puts it this way:

Grant a simple archetypal creature, like the mud-fish or Lepidosiren, with the five senses & some vestige of mind, & I believe Natural Selection will account for production of every Vertebrate animal. (Darwin to Lyell, 20 October 1859 [CCD 7: 354])

But the "argument" of the Origin is not simply one in favor of descent with modification. To establish his own conclusion Darwin must show that his explanation of the beautiful and wonderful adaptedness of organic beings preferable to its competitors. The most obvious competing explanation is the hypothesis of intelligent design; and we know that Darwin was well up on the natural theology of his time and that he held William Paley's Natural Theology (1802) in very high regard. We should expect Darwin, then, to articulate reasons for thinking his own account preferable to Paley's.

This is precisely what Darwin does in the Origin. After making a pretty straightforward statement of his theory in the first half of the book, Darwin devotes the second half to showing that his theory offers a better explanation of the data than does "the theory of creation." Darwin and Paley agree that certain facts require explanation, e.g. the adaptedness of organic beings to their surroundings. They disagree over the sort of explanation to be offered. Paley's explanation invokes the designs of an intelligent being: Paley's claim is that without intelligence there can be no adaptedness of means to ends, no purposiveness, no intelligible order in nature -- just as the existence of a watch on a heath requires the existence of a watchmaker. But the explanation Darwin suggests in the Origin -- "descent with modification" -- doesn't posit a watchmaker-like intelligent designer. (It doesn't rule out such a designer, of course; it simply makes no explanatory use of such a being. In the letter to Lyell cited above, Darwin writes "I have reflected a good deal on what you say on necessity of continued intervention of creative power. I cannot see this necessity; & its admission, I think would make theory of nat. select. valueless.") But how can Darwin show his explanation to be superior to Paley's?

By beating Paley at his own game. Natural Theology is methodologically quite sophisticated. Paley begins by adducing a number of very striking facts about organisms, their parts, and their relations with each other. He suggests that there must be some explanation of these facts -- there must be an answer to the question why things are this way. It can't simply be a coincidence, e.g., that the vertebrate eye is so well adapted to see; the odds against are just too great. Chance, then, is no explanation. Paley then goes on to eliminate other alternatives to the intelligent designer hypothesis. The gist of his argument is simple:

5. Certain things are true about organisms, e.g. that they are well suited to their surroundings, that many of them possess organs well adapted to performing valuable functions, etc.;

6. There must be some explanation of these facts;

7. The best explanation is one which invokes the designs of an intelligent being;

Therefore

8. We have reason to think such a being exists.

Darwin agrees with (5) and (6), but differs with Paley over (7). In fact, (7), far from being true, is for Darwin the reverse of the truth. Not only is Paley's designer hypothesis not the best explanation of the facts cited in (5), it fails to explain some of the most striking facts about organisms and their surroundings. It is these facts, and the inability of the designer hypothesis to account for them, that Darwin seizes on in the second half of the Origin. Darwin's suggestion is that his own theory of descent with modification accounts for evident facts about organisms and their surroundings that are inexplicable on the designer hypothesis.

In the concluding chapter of the Origin, in his "recapitulation" of his argument, Darwin reviews some of the facts that he believes are anomalous on "the theory of creation," but explicable on the hypothesis of descent with modification. (A brief Darwinian "explanation" appears in square brackets, but obviously all of these are controversial; read the rest of the Origin for the details.).

1. No "mark of demarcation" can be drawn between species (supposedly produced by special acts of creation) and varieties (allowed to be the result of secondary laws). [The difference between species and variety is conventional: species are nothing but "strongly marked and permanent varieties."]

2. We usually find many varieties within species in regions where many species of a genus have been produced. [Speciation -- the formation of new species within a genus -- is simply the production of varieties within a species on a larger, longer scale.]

3. Species of large genera retain the character of varieties, while species of small genera don't: (i) they differ from each other less than do species of smaller genera; (ii) closely allied species of larger genera tend to cluster in small groups around other species. [Large genera are large in part because their members vary and diverge from each other to fill available niches. Liability to vary is itself heritable.]

4. Forms of life are arranged in groups subordinate to groups, all in a few great classes. [The tendency of large groups to increase in size and diverge in character, together with much extinction, accounts for this.]

5. "Natura non facit saltum" -- nature is prodigal in variety, niggard in innovation. [Natural selection acts by accumulating very slight favorable variations.]

6. We find strange animal-surroundings pairs, e.g. birds of woodpecker-like form feeding on ground insects; upland geese which rarely swim, yet have webbed feet; thrushes which dive and feed on sub-aquatic insects. [Natural selection adapts slowly-varying descendants of each form to any unoccupied or ill-occupied niche. Perfection isn't necessary, only competitive advantage.]

7. We find occasional reversions to long-lost characters, e.g. occasional appearance of stripes on shoulders/legs of several species of the horse genus. [These species have descended from a striped ancestor; in general the characters of distant ancestors occasionally display themselves in living forms.]

8. Specific characters -- which distinguish species within a genus -- are more variable than generic characters -- which distinguish between genera. [Species are well-marked varieties; they have already varied since they branched off from the common ancestor in a certain character, by which they have come to be specifically distinct from each other. The same character would be more likely still to be variable than the generic ones, which have been inherited without change for a very long period.]

9. Parts developed in an unusual manner (e.g. trunk, tail, beak) in any one species of a genus are especially liable to variation. [Since the several species branched off from the common ancestor, this part has undergone an unusual amount of variability and modification -- witness its unusual shape, etc. We should expect this to continue.]

10. Allied species in considerably different conditions of life follow nearly the same instincts. [Instincts change by successive slight profitable modifications; all species of a genus descended from a common ancestor, and so inherit much in common.]

11. Some instincts are apparently not perfect, and are liable to mistakes (e.g. the moth and the flame); many instincts cause other animals to suffer. [See explanations of (f) and (j).]

12. Offspring of different species which are crossed (i.e. hybrids) follow the same complex laws in the degrees and kinds of resemblance to their parents as do the crossed offspring of acknowledged varieties -- i.e. crosses among species show the same laws of inheritance as crosses among varieties. [See explanations of (a) and (b).]

13. The fossil remains of each formation are intermediate in character between fossils in formations above (more recent) and below (older). [They occupy intermediate positions in chains of descent.]

14. All extinct beings belong to the same system as recent beings, and fall either into the same or into intermediate groups. [See (m).]

15. The more ancient a fossil is, the oftener it stands in some degree intermediate between existing and allied groups. [The descended groups have diverged in character, so the ancestor will appear intermediate compared to its later descendants.]

16. Recent forms appear "higher" or "more developed" or "better adapted" or "more specialized" than ancient or extinct forms. [The later, better adapted forms, conquered the older ones.]

17. We find a striking parallelism in (i) the distribution of organic beings throughout space (e.g. similar niches filled with similar organisms), and in (ii) the geological succession of organic beings throughout time (e.g. similar developments at similar periods). [Beings in a particular region are related by ancestry, and the means of modification have remained the same. Given migration from one part of the world to another we should expect parallelisms of this sort.]

18. On the same continent, under the most diverse conditions, most of the inhabitants within each great class are related. [They are lineal descendants of a common ancestor.]

19. Oceanic islands are inhabited by a few species; many of these species are peculiar to these islands; animals which can't traverse wide spaces of ocean (frogs, terrestrial mammals) don't inhabit oceanic islands, yet other new and peculiar species (e.g. bats, birds) are found. [Migration with subsequent modification; see (q) and (r).]

20. Whenever many closely allied species inhabit two areas (e.g. two of the Galapagos Islands), some identical species common to both still exist; and whenever many closely allied but distinct species occur, many doubtful forms/varieties occur as well. [The same parents both once inhabited both areas; the inhabitants of each area are related to the inhabitants of the nearest source from which immigrants might have been derived.]

21. All past and present organic beings constitute one grand natural system, with group subordinate to group, and extinct groups often falling between recent groups. [Specific differences are the result of most recent divergences; genera next, etc. "The real affinities of all organic beings are due to inheritance or community of descent" (479).]

22. Some characters are more serviceable than others for classification -- e.g. adaptive characters, though important for the organism, are unimportant in classification; characters derived from rudimentary parts, though of little importance for the organism, are often very valuable in classification; embryological characters are most valuable. ["The natural system is a genealogical arrangement, in which we have to discover the lines of descent by the most permanent characters, however slight their vital importance might be." (479)]

23. We find homologies -- e.g. the framework of bones is the same in a human's arm, a bat's wing, a porpoise's fin and a horse's leg. [Each structure represents the gradual modification of parts/organs which were alike in the original ancestors of the class.]

24. The embryos of humans, birds, reptiles and fish are alike; and each is unlike its adult form. [Successive variations don't always appear at an early age; the differences appear at later stages in the development of the embryos into mature forms. The common ancestral form is more evident in embryo than in adult.]

These are some of the evident facts which Darwin claims are inexplicable on the designer hypothesis. His own theory, however, explains them -- they are just what you would expect if his theory is true. The logic of Darwin's argument is precisely that of Paley's:

5. Certain things are true about organisms, e.g. (A)-(X);

6. There must be some explanation of these facts;

But instead of Paley's (7) Darwin suggests

9. The hypothesis of descent with modification explains (A)-(X) more satisfactorily than does the designer hypothesis;

from which it follows that

10. The hypothesis of descent with modification is to be preferred to the designer hypothesis.

This is the conclusion of the "one long argument" presented in The Origin of Species. As Darwin was to put it in his Autobiography, written late in his life (1876),

The old argument of design in nature, as given by Paley, which formerly seemed to me so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows. (Autobiography, ed. Nora Barlow [Norton, 1958], 87.)