Fact: current life forms differ from those of previous times,
          and are 'descended' from them.
   Evolution is descent with modification.
                  ["Descent" in the genealogical sense]

 Fact: current life forms are extremely variable,
          both within and among species.
   Evolution accounts for variation among living organisms.
                  [Variation occurs in time and space]

"Theory of Evolution"
      The explanation of observed patterns of temporal & spatial variation
            in terms of biological and physical processes.

     Natural Selection provides a mechanism for Evolution:
            Modern evolutionary theory seeks to clarify this mechanism.

      The observable order in Nature is due to common descent from an ancestor:
            Organisms resemble each other because they are related.

Fact: All living things are related (the basic fact of biology):
      The degree of relationship provides a basis for "natural classification":
            Taxonomy should reflect the phylogeny of organisms.

I summon the supernatural beings
Who first contrived
The transmorgifications
In the stuff of life.
You did it for your own amusement.
Descend again, be pleased to reanimante
This revival of those marvels.
Reveal, now, exactly
How they were performed
From the beginning
Up to this moment.
                                        Ovid: Metamorphoses (trans. Ted Hughes)

     The Classical Tradition: Plato & Aristotle
       Theory of Forms (essences, eidos)
                  'real' objects are manifestations of 'ideal' forms
                  variation is illusory  [see Plato "The Republic"]
       Dichotomy: the world is composed of paired opposites
                  "A" versus "not A" classes
                  good / bad, right / wrong, up / down, light / dark, male / female, etc.
                      e.g., vertebrates vs. invertebrates
       Aristotle (384-322 BCE) - "Father of Biology"
                Five books on zoology ("Generation of Animals")
                Biological structures have purpose: Efficient versus Final Causes

     Natural Theology (John Ray 1627-1705)
            "The Wisdom of God, Manifested in His Creation"  (1691)
            'Ideal' forms exist in the Mind of God:
                  'real' world created by God (Genesis 1:1)
                        The study of nature is a pious activity

         Scala Naturae: the "Great Chain of Being"
                  Creation is an infinitely graduated progressive series
                  Time scale is short (ca. 6,000 years)
                  Species are static: no new forms, no change, no extinction

     Linnean Taxonomy (Carl von Linne [Carolus Linnaeus] 1707-1778)
            "Systema Naturae" (1735; 10th ed. 01 January 1758)
                  4,162 animals described
          binomial nomenclature: genus + species names
                  Worked "Ad majorem Dei gloriam": for the greater glory of God

     Exploration creates a Scientific Crisis
       New forms are discovered that don't fit the Scala
       Extinctions have evidently occurred
       Variation is real in space: what about over time?

The Darwinian Revolution

     Charles Darwin (1809-1882)
            BSc (Cambridge): pre-med
            Naturalist on board HMS "Beagle" (1831-36)
                "The Voyage of the Beagle" (1839) a best-seller
            Read Robert Malthus "On Population" (1838):
                population increases exponentially, resources increase arithmetically
            [Married Emma on 29 Jan 1839]
            Letter from Alfred Wallace (1823-1913) in June 1858

Observation: In any species, more young are born than can possibly survive.

Observation: Yet a species' numbers do not increase without limit.

CONCLUSION: There is a Struggle for Survival,
        and differential survival & reproduction occur within species.
        [Darwin: "I use 'struggle' in a large and metaphorical sense..."].

Observation: Individuals within species show variation
          that affects the probability that they will survive this struggle and leave offspring.

CONCLUSION: Those individuals that survive and reproduce do so in consequence
      of their "adaptively superior" variation (they are "more fit")
      This process of differential survival and reproduction is called Natural Selection.

Observation: Variation is heritable: offspring tend to resemble their parents.
            [Remember that Mendelian genetics was unknown in 1859).

CONCLUSION:
       Adaptively superior variation will be inherited by the offspring generation.
      That is, evolution occurs as descent with modification.

For further reading:

    Loren Eisley (1959). "Darwin's Century." Doubleday.
    William Irvine (1955). "Apes, Angels, and Victorians: Darwin, Huxley, & Evolution." McGraw-Hill.
    Ernst Mayr (1994). "One Long Argument". Harvard University Press.
    Gordon Ratray Taylor (1963). "The Science of Life." McGraw-Hill.