This is a must-have for world builders. It gives a broad but thorough look at how creatures can be adopted or survive in a multitude of settings and environments. Even if you're using an established setting, the information provided here can help you provide a large dose of the unexpected and do so with a better explanation than "Because I said so." Even in a fantasy setting, suspension of disbelief only goes so far. This book helps get more mileage out of it, largely by not requiring it in the first place.
[5 of 5 Stars!]