An icthys is what's commonly known as a "Jesus fish". It is commonly seen on bumper stickers of professed Christians. The origins of the symbol are dubious, conflicted, and confusing: I'll just let you read the Wikipedia article on this one: http://en.wikipedia.org/wiki/Icthys.
Mentioned in the article is the common "Darwin fish" parody, but not mentioned is the sometimes seen variation of a "Jesus fish" eating a "Darwin fish". I'm not completely sure what the intended meaning of this is. While I suppose it might be intended as belief in Jesus overpowering belief in Darwin, but to me it seems a more fitting representation to how some branches of Christianity mix the two, effectively absorbing evolution into said branches. Of course, it's the conflict between the two and the debates springing from this that led to Flying Spaghetti Monsterism,
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