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definition of a species

What is the definition of a species? Is it an organism that cannot produce
fertile offspring with another species? How can you tell if that is the case by
looking at fossils? What about eg. lion/tiger crosses (ligers and tigons)? Are
they sterile and therefore not new species? How do we know that fossil animals
also were not sterile cross-breeds? And what about dogs (Canis domesticus?). 
Aren't they one species? Yet if we found the fossil remains of a Great Dane and
a Chihuahua, would we have classified them as one species? Would this much
variation be encountered naturally (esp. in dinosaurs), or is this strictly the
result of mankind manipulating a species' breeding. I'm confused (obviously)!

Scott Horton
Geophysicist/Computer Programmer