Creationist biologists may have rejected species fixity for many decades, but the Creationists MOVEMENT certainly promoted it, and still does quite widely.Unfortunately, much of the follow-up comments have gotten lost in the minutiae, since I think WebMonk is quite right in this basic claim. The wider creationist movement does indeed accept species fixity, and the major creationist organizations have only recently begun trying to fix that problem.
To show you why I think so, I'm first recommending reading over my paper on species variation and creationism [PDF], particularly the part about antievolutionists. Describing them, I wrote,
These individuals simply rejected everything that Darwin argued and vigorously defended the creationist caricature from Origin of Species, species fixity and all, as though they were themselves suffering a personal assault.It is from this tradition that we get modern advocates of species fixity.
To show you how this has happened, I'm going to "quote mine" some books. I fully acknowledge that the quotes that I select may not fully represent the authors' positions, but that's part of my point. Many times creationists are not entirely consistent in their public presentations. They'll deny any evidence of evolution including speciation, but then they'll turn around and acknowledge that some limited variation may have produced a few species here and there. Listeners and readers eager to find evidence against evolution are not listening to the discussion of speciation within creationism, they instead latch onto claims of "no evidence of speciation."
Let's begin with Leander Keyser's 1926 The Problem of Origins. Keyser was a prominent Lutheran theologian in those days. On p. 74, he wrote:
Moreover, the biblical account of distinct creations agrees with what we see in the organic realm to-day; namely, each species breeding true to form
Famous antievolutionist Harry Rimmer wrote in his The Theory of Evolution and the Facts of Science (I'm quoting from the 1954 edition, p. 43),
By the multiplied evidences that prove the unbridgeable gap between various kinds of cells, establishing the foundational reason for the phenomenon of fixity of species, we can say we are in a new day of biology.
In Richard Culp's Remember thy Creator, published by Baker Book House in 1975, he wrote (p. 185),
It is almost idle to suggest that, while the phenomena mentioned under "speciation" certainly do exist as causes for variation, there is not the slightest evidence that these have resulted in new species.
Even the Biblical Creation Society got into the act (sorry, Paul). Their first monograph, titled "The Species Problem," was written by Edward Brearley in 1980. He doesn't come right out and advocate species fixity, but it sure sounds like he grudgingly accepts very, very limited speciation (pp. 22-23),
Genesis tells us that, after Creation of many different kinds of organisms, plants and animals brought forth after their 'kind'. 'Kind' may be a bit difficult for us to define taxonomically, and here and there may include more than one currently recognised species. ... Species are the means whereby the Creator sustains the organisms in His universe in all their beauty and diversity. The genetic systems maintaining the species were created and are sustained by Him too.
In 1999, Rose Publishing put out an anonymous pamphlet titled "Creation & Evolution," which I purchased only a few years ago at a Christian bookstore in Lansing, Michigan. On p. 7, in bold type, it says,
While accepting variation within species (microevolution, not Darwinian evolution), most creationists do not believe that new species have arisen through macroevolution.
Another book I purchased at a Christian bookstore just a few years ago is The Evolution Handbook, a massive, nearly 1000-page tome written by Vance Ferrell and originally published in 2001. (I think my edition is 2005. Like everything else in the book, the publication information is confusing.) There are apparently tens of thousands of copies of this book out there. Chapters 9, 10, and 11 all deal with species fixity. Chapter 9 is about "why natural selection only makes changes within species." Chapter 10 is about "why mutations cannot produce cross-species change," and Chapter 11 purportedly answers the question "why the species barrier cannot be broken."
Still not convinced species fixity is alive and well even today? How about Ron Rhodes's 2004 The 10 Things You Should Know about the Creation vs. Evolution Debate? Chapter six is titled "Mutations and Natural Selection Cannot Bring about New Species."
You might be thinking that these are all obscure or old works that were not heavily promoted by mainstream creationist organizations like AIG, ICR, and CMI. You'd be right about that. How about some quotes from books you've heard of that were sold by these organizations even quite recently?
In The Evolution of a Creationist, Jobe Martin (the guy from the Incredible Creatures that Defy Evolution videos) wrote (pp. 40-41),
Darwin had no idea how one species might change into another. ... Scientists today remain as baffled as Darwin. ... This, then, is the really big question of evolution: How does it happen? God says He created each thing "after its kind".
In A.E. Wilder-Smith's 1981 book The Natural Sciences Know Nothing of Evolution, we read (pp. 142-143),
If for any reason any ecological niche has not been already colonized, then a mutation may possibly be able to contribute to its colonization. Darwin's finches provide us with a good example of the conquest of a new ecological niche by the abovementioned mechanism. But species boundaries are not violated by this change.
Finally, from Ian Taylor's In the Minds of Men: Darwin and the New World Order (1991 edition), we read (p. 176),
The wide variation possible within a species which can bring variants to the point of becoming reproductively isolated, has been called by some micro-evolution, but the only new species which has been created has been on paper by the term of the definition.
I realize that these quotes are ripped from their context and probably do not represent the complete position of the authors, but that's part of my point. Careless readers looking at these quotes conclude that creationists must believe that there are no new species, hence that species are fixed. WebMonk is not at all incorrect in assuming that until recently even mainstream creationist organizations have advocated both positions through their book sales: speciation does not contradict biblical creationism and species are basically fixed. Thankfully, in the past twenty or so years, creationists have worked aggressively to correct this inconsistency, although it still lingers on the fringes of the creation movement and on the internet.
On the other hand, as Paul Garner, his other commenters, and I have documented, speciation has a long tradition in creationism stretching back even before Darwin. I also think Paul's critique of Denis Alexander is right on the mark. For Alexander to equate young earth creationism with species fixity is preposterous and irresponsible. It's a classic straw man argument. It would be nice if our critics actually criticized the things we really believe. Species fixity has not been a part of real creationism for decades. Get with the program.