His failure to see the limits of variation of species
Darwin got the idea about natural selection in part from observing artificial selection. For instance, he noted the way pigeon breeders came up with a great variety of pigeons. Yet we should remember, they are still all classified as pigeons!
He thought that from this variety, given enough time, pigeons could eventually evolve into some other type of birds, such as eagles or vultures, and gradually, even to other creatures such as mammalian bats.
No one seriously disputes the notion of "change over time" in biology—heredity sees to that. We vary from our parents and grandparents—but that is not what the theory of evolution is all about. It is really an attempt to explain how microorganisms, insects, fish, birds, tigers, bears and even human beings actually became what they presently are through the passage of time.
There is also no problem accepting what is called microevolution, or change within a species, where mutation and natural selection do play a role. We have examples in nature of these minor adaptations within organisms, such as microbial antibiotic resistance, modifications in the fruit fly’s eyes and wings and the varying beak sizes of finches. But it’s crucial to note that these microbes are still microbes, the fruit flies are still fruit flies and the finches are still finches!
Darwinian evolution—what is taught in the schools—is about macroevolution, or changes beyond the limits of the species kind to create another distinct species. It consists of three suppositions: 1) all living things descend from a common ancestor; 2) the principal mechanisms for the changes are natural selection and mutation; and 3) these are unguided, natural processes with no intelligence at work behind them.
But have we seen either in present life forms or in the fossil record that creatures are slowly changing and mutating from one kind to another? Never.
As biochemist and agnostic Michael Denton states: "The fact is that the evidence was so patchy one hundred years ago that even Darwin himself had increasing doubts as to the validity of his views, and the only aspect of his theory which has received any support over the past century is where it applies to microevolutionary phenomena.
"His general theory, that all life on earth had originated and evolved by a gradual successive accumulation of fortuitous mutations, is still, as it was in Darwin’s time, a highly speculative hypothesis entirely without direct factual support and very far from that self-evident axiom some of its more aggressive advocates would have us believe" (Evolution: A Theory in Crisis, 1985, p. 77).
Zoologist Pierre Grasse, the late president of the French Academy of Sciences, boldly stated that these adaptations "within species" actually have nothing to do with evolution. They are mere fluctuations around a stable genotype—a case of minor ecological adjustment. He compared these changes to a butterfly flying within the confines of a greenhouse, being able to fly only so far before it has to turn sideways or back.
Darwin hoped future research and discoveries would show that the more than a million species on the earth today or the millions of extinct animal fossils would reveal some gradual transition between them. His lack of understanding the laws of inheritance and the solid genetic barriers that were discovered between species has undermined his case.