True in 1875, true today:

Those who cavalierly reject the Theory of Evolution as not being adequately supported by facts, seem to forget that their own theory is supported by no facts at all. Like the majority of men who are born to a given belief, they demand the most rigorous proof of any adverse belief, but assume that their own needs none. —Herbert Spencer

I commented on just this tendency in “Intelligent design’s double standard“; for example, compare this gem from Michael Behe’s testimony at the Dover trial:

[To believe complex biochemical systems arose through Darwinian processes, n]ot only would I need a step-by-step, mutation by mutation analysis, I would also want to see relevant information such as what is the population size of the organism in which these mutations are occurring, what is the selective value for the mutation, are there any detrimental effects of the mutation, and many other such questions. [Kitzmiller v. Dover, day 12]

with his response, the previous day, to a request that he identify the mechanism by which intelligent design explains the origins of complex biological structures:

Again, [intelligent design] does not propose a mechanism in the sense of a step-by-step description of how those structures arose. But it can infer that in the mechanism, in the process by which these structures arose, an intelligent cause was involved. [Kitzmiller v. Dover, day 11]

My goodness, how the standard of evidence changed in one day! To believe that complex structures could be explained by a process for which (as Dr. Behe acknowledges) massive piles of evidence exist, he requires an impossible level of detail: “a step-by-step, mutation by mutation analysis,” population sizes, selective coefficients, pleiotropic effects, and “many other such questions,” all for something that happened (if we’re talking about the bacterial flagellum) billions of years ago. But for a supernatural process that has never been observed, a step-by-step description is an unfair standard.

This kind of thing is exactly what Spencer warned against 130 years earlier*:

Should the believers in special creations consider it unfair thus to call upon them to describe how special creations take place, I reply that this is far less than they demand from the supporters of the Development Hypothesis [descent with modification]. They are merely asked to point out a conceivable mode. On the other hand, they ask, not simply for a conceivable mode, but for the actual mode. They do not say – Show us how this may take place; but they say – Show us how this does take place. So far from its being unreasonable to put the above question, it would be reasonable to ask not only for a possible mode of special creation, but for an ascertained mode; seeing that this is no greater a demand than they make upon their opponents.

To be fair, Dr. Behe, like most intelligent design advocates, is not arguing for “special creations”; he has said many times that he does not doubt common descent. The point here is that Spencer’s argument applies just as well to intelligent design: for natural selection, a process that undeniably works, a level of detail that would be impossible to obtain is demanded. However, when considering supernatural intervention, which has never been shown to occur, the particulars are suddenly dispensable.

Spencer, H. 1875. The development hypothesis. In: Illustrations of universal progress: A series of discussions (H. Spencer, ed), pp. 377–383. New York: D Appleton & Company. doi: 10.1037/12203-009 (free Kindle edition: https://amzn.to/2yat4fD)

*Longer, actually. The 1875 book chapter was a nearly word-for-word reprinting of his 1852 article in The Leader. Yes, you read that right, seven years before Darwin published The Origin of Species. The idea of common descent long preceded Darwin.