Darwin's theory of evolution was motivated and justified by on-going theological and philosophical concerns. Though these concerns were spread across different continents, religions and centuries, there is a particular religious perspective at work. And naturally it is from this perspective that evolutionists argue. Their religious arguments are not merely sidebar rebukes of creation.
Consider, for example, Darwin's argument that the pattern, or classification, which the different species fall into would never have resulted if they had been created:
The several subordinate groups in any class cannot be ranked in a single file, but seem clustered round points, and these round other points, and so on in almost endless cycles. If species had been independently created, no explanation would have been possible of this kind of classification.
This is, of course, a metaphysical claim. If the species were independently created then, so the reasoning goes, they would have no pattern of similarities. After all, independently created species must have independent designs. And since we find similar designs, the species must not be created. Instead, they must be derived one from the other. For there can only be two possibilities: created with independent designs or evolved with design similarities.
It is not that the species obviously morph into other species, but rather that they do not appear to have been created. The reasoning, as always, is metaphysical.
And this metaphysical claim is distinctly evolutionary. It is not representative of design or creation. Indeed, Linneaus had constructed his hierarchy revealing the classification described by Darwin. But Linneaus believed god created the hierarchical classification. For Linneaus, independent creation certainly did not imply independent design. And this is no trivial example—Linneaus was the leading scientist of his day, and his views were tremendously influential. The evolutionist Darwin was not so much addressing creationism as it was commonly understood, but rather creationism as it was understood by Enlightenment theology.
Nor was Darwin's religious claim a trivial example. It has been repeatedly proclaimed ever since. As Niles Eldredge more recently explained:
Could the single artisan, who has no one but himself from whom to steal designs, possibly be the explanation for why the Creator fashioned life in a hierarchical fashion—why, for example, reptiles, amphibians, mammals, and birds all share the same limb structure?
Of course not. To say that the Creator came up with such a pattern makes no sense according to the evolutionists. Evolution is driven by religious belief. This world is not what we would expect from a creator or designer, and so evolution is mandated.
If this is how evolutionary thought works, then we would expect evolutionists to make truth claims about their theory. If my beliefs dictate that this world is not created or designed, then I will conclude it arose naturalistically, on its own.
Therefore we can make a prediction about evolutionary thought. If it is driven by religious beliefs, then we should find these sorts of truth claims in the literature. This prediction has been fulfilled a great many times over. It would be difficult to tally up the multitude of such truth claims made by evolutionists, both before and after Darwin, in addition to Darwin himself.
The eighteenth century Scottish philosopher David Hume, for example, argued against design. His antagonist Cleanthes made a powerful design argument. The protagonist Philo admitted the argument was a great challenge for him, but it was neutralized by the evil in the world. “I needed all my skeptical and metaphysical subtlety to elude your grasp,” admitted Philo, but “Here I triumph.”
Hume's triumph was not merely that Cleanthes’s metaphysics were wrong. It was that the evolutionary metaphysics of naturalism were right.
Likewise when Darwin argued against his version of special creation he did not merely conclude against that particular view. He believed that that version of special creation was the version of special creation. We might say he believed in that view. If god created the world, then it wouldn't look this way. Therefore evolution must be true. Darwin's religious beliefs allowed him to make a far more comprehensive conclusion. Here is an example:
We cannot believe, that the similar bones in the arm of the monkey, in the fore-leg of the horse, in the wing of the bat, and in the flipper of the seal, are of special use to these animals. We may safely attribute these structures to inheritance.
Darwin does not merely conclude against his particular brand of creation. He concludes against creation in general. Creation must be false, period. And so inheritance must be the explanation.
This method of metaphysical reasoning runs all through the evolution genre. Evolutionists consistently claim their theory is an undeniable fact. And they prove it to be so. But their proofs are always religious.
Pseudogenes for example are sometimes found to be disabled by identical mutations in cousin species. In typical fashion evolutionist Jerry Coyne concludes they wouldn't have been designed that way and that “Only evolution and common ancestry can explain these facts.” [Why Evolution is True, 68]
This is an example of how this religious reasoning is often in the form of an if-and-only-if statement. In this case, if and only if evolution is true, then we would observe such identical mutations in pseudogenes. Here is another example of this non scientific logic:
One of my favorite cases of embryological evidence for evolution is the furry human fetus. We are famously known as "naked apes" because, unlike other primates, we don't have a thick coat of hair. But in fact for one brief period we do--as embryos. Around sixth months after conception, we become completely covered with a fine, downy coat of hair called lanugo. Lanugo is usually shed about a month before birth, when it's replaced by the more sparsely distributed hair with which we're born. ... Now, there's no need for a human embryo to have a transitory coat of hair. After all, it's a cozy 98.6 degrees Fahrenheit in the womb. Lanugo can be explained only as a remnant of our primate ancestry: fetal monkeys also develop a coat of hair at about the same stage of development. Their hair, however, doesn't fall out, but hangs on to become the adult coat. And, like humans, fetal whales also have lanugo, a remnant of when their ancestors lived on land. [Why Evolution is True, 80]
The religion that motivates and justifies evolution is not an afterthought response to those pesky creationists. It is front and center. It is at the inner core of evolutionary thought. It is what makes evolution a fact. As Stephen Jay Gould explained:
Odd arrangements and funny solutions are the proof of evolution—paths that a sensible God would never tread but that a natural process, constrained by history, follows perforce. No one understood this better than Darwin. Ernst Mayr has shown how Darwin, in defending evolution, consistently turned to organic parts and geographic distributions that make the least sense.
Evolution is not about science. But when a mirror is held up and evolutionists are confronted with their own words, they suddenly cry foul. Like Captain Renault, they are shocked, shocked to find religion is going on in here. Evolutionists are their own judge.