Contrary to the statements of most evolutionary writers, few (if any) creationists have ever advocated the idea of absolute fixity of species. The Biblical unit of biological taxonomy, of course, is the Genesis "kind" (Hebrew, min).

It may be worth mentioning that this fact was stressed in my first book on creationism some 55 years ago:

We have noted from the statements of leading evolutionary scientists that no true vertical evolution from one kind of organism to a more complex kind has ever been observed in all human history.

"And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure" II Cor. 12:7.