Well consider this...

Imagine if ducks were mammal. Yes, take your time, close your eyes and concentrate. What makes mammals... mammals? Now, think about ducks. Common, mundane ducks. Then combine these images in your head.

What is your first thought when you succeed at this? "Thanks god they aren't real"? Well, it should be "Thanks evolution they aren't real" instead. It is because ducks never mutated into that horrible abomination from your imagination that we can enjoy the sight of those aquatic birds.