Although much has been written about the theological conflicts with Darwinian theory, little is known of the powerful scientific objections that modified Darwin's beliefs.

During Darwin's lifetime, the accepted theory of heredity was not Mendel's theory of particulate inheritance, which, though published, was unrecognized, but the theory of blending inheritance, which holds that forms intermediate between those of the parents result from mating. Jenkin pointed out that if a rare and favorable mutation occurred, it would soon be blended out by repeated crossings from the wild-type form. Disputing Darwin's conception of evolution as proceeding through the natural selection of those with slightly better characteristics that arose randomly, Jenkin concluded that natural selection could not account for the tremendous diversity of life, hypothesizing that large numbers of organisms mutated simultaneously in the same direction-a controlled orthogenetic process resembling a series of "special creations."

Since "special creationism" was an ideological target of his, Darwin found himself in a quandary. Although he did not abandon his theory, he admitted that natural selection played a much smaller part in evolution than he had previously claimed. He also embraced the Lamarckian concept that acquired traits in parents are transmitted to their offspring, thus providing a mechanism by which an entire population could change in the same direction at once.

Another potent objection came from the physicists led by Lord Kelvin, who contested the assumption of previous geologists and biologists that life had existed for billions of years, if not infinitely. How, they asked, could evolution proceed by slow steps in millions of years, and how could advanced forms recently evolved show such great differences? The Kelvinists, basing their conclusion on the assumption that the sun was an incandescent liquid mass rapidly radiating heat, calculated that the age of the earth was between 20 and 40 million years.

Admitting that their calculations were correct and their premises rational, Darwin was forced to adjust this theory. He proposed that change had occurred much more rapidly in the past than in the present, where species seemed static, and that more advanced forms varied more rapidly than lower forms. This provided further reason to advocate Lamarck's theory of inheritance, because that could account for the rapid change.

Interestingly, both these retreats of Darwin were later shown to be faulty. The discovery that the sun runs on a nearly infinite amount of atomic fuel totally invalidated Kelvin's argument, Mendel was "rediscovered" in the twentieth century, when it was pointed out that the particulate nature of inheritance meant that favorable mutation not only could persist, but could rapidly become prevalent.