Darwin’s Theory Takes Form

                        A.Old Bones and Armadillos

                              1.Darwin returned after five years at sea and began
                                pondering the "species problem"–what could explain the
                                remarkable diversity among organisms?

                              2.In Argentina, Darwin had observed extinct glyptodonts
                                that bore suspicious resemblance to living armadillos;
                                Darwin wondered if the present species had evolved from
                                the extinct one?

                        B.A Key Insight–Variation in Traits

                              1.Thomas Malthus had suggested that as a population
                                outgrows its resources, its members must compete for
                                what is available; some will not make it.

                              2.Darwin felt that if some normally variant members of a
                                population bore traits that increased their survival, then
                                nature would select those same individuals to survive,
                                reproduce, and possibly change future populations’ traits.
                                   a.On the Galapagos Islands, the dozen or so species
                                     of finches all varied from one another to some
                                     extent but resembled the mainland finches to some
                                     degree also; perhaps they had descended from
                                     common ancestors.
                                   b.Darwin reasoned that a population is evolving when
                                     its heritable traits are changing through successive
                                     generations.

                              3.In 1858, Darwin received a paper from Alfred Wallace,
                                who had developed much the same theory on natural
                                selection but independently of Darwin.

                              4.Darwin and Wallace presented a joint paper but Darwin
                                published (alone) his ideas in book form in 1859

                       C. Regarding the First of the "Missing Links"

                        A.Darwin saw evolution of one kind into another as happening
                           gradually, in small increments, over hundreds or thousands of
                           generations.
                        B.The possibility of transitional forms was illustrated when, in
                           1861, fossil evidence of Archaeopteryx was unearthed.
                              1.This animal appears to be a transitional form between
                                reptiles and birds.
                              2.Like birds, it was covered with feathers; but like reptiles,
                                it had teeth and a long, bony tail.