No Clear-Cut Boundaries in Nature
Creationists often talk about the gaps in the fossil record, as if evolution is a lie because we don't have any skeletons that are half-man, half-chimp. I think the best response to that is, Of course there are gaps in the fossil record. The vast majority of organisms never become embedded and preserved in the earth's crust.
In an interesting column in the L.A. Times, David P. Barash, a professor of psychology at the University of Washington, writes,
Yearn as we might for clear-cut, yes-no boundaries, nature only rarely obliges. There aren't many genuine leaps in the biological world, an observation that contributed greatly to Charles Darwin's insight about the gradual transformation of species.
The essay isn't really about evolution, but it does have an interesting look at how people often search for a biological basis to an argument, even where none exists.
Thanks to ejswanso for the tip.