His theory of the tree of life
The only drawing Darwin had in his book The Origin of Species is that of the supposed "tree of life." It pictures the imaginary transformation of a common ancestor (at the root level) into the different species we see today (at the twig level). Yet the drawing is actually based on slight variations within a species after many generations, and then he adds some suppositions.
Again Darwin went well beyond the evidence. He took limited evidence about adaptations and extrapolated it to the idea that a species or genus (group of interbreeding species) can transform into a completely different one—all based on speculation. He cleverly said, "I see no reason to limit the process of modification, as now explained, to the formulation of genera [plural of genus] alone" (p. 121). He had to say this since no more direct evidence was forthcoming!
As Jonathan Wells notes: "The most fundamental problem of evolution, the origin of species, remains unsolved. Despite centuries of artificial breeding and decades of laboratory experiments, no one has ever observed speciation (the evolution of a species into another species) through variation and selection. What Darwin claimed is true for all species has not been demonstrated for even one species" (The Politically Incorrect Guide to Darwinism and Intelligent Design, 2006, p. 64).
So instead of a "tree of life" that begins with one or a few common ancestors and then branches out, there is actually an inverted and quite divided "tree of life," where the branches of life were very diverse and numerous at the beginning. Through extinction and sudden appearances, we have fewer kinds of life-forms today than in the past.
"Of all the icons of evolution," adds Dr. Wells, "the tree of life is the most pervasive because descent from a common ancestor is the foundation of Darwin’s theory…Yet Darwin knew—and scientists have recently confirmed—that the early fossil record turns the evolutionary tree of life upside down. Ten years ago it was hoped that molecular evidence might save the tree, but recent discoveries have dashed that hope. Although you would not learn it from reading biology textbooks, Darwin’s tree of life has been uprooted" (ibid., p. 51).