Snail, I have also heard that fixity of species was a common view in the past . This was of course easily shown to be false , and as a result evolution gradually made gains. Creationism has not taught fixity of species for a long time , if ever. Darwin seems to have got his ideas from others before him including creationist writings. Darwin, thought that the variation between species that developed could be extrapolated to posit a complete tree of life encompassing all organisms, as opposed to the biblical teaching , that was observationaly attested , that they reproduced "after their kind ". Now, was it logical to assert that which was not observable ?