Eucalyptus globulus and others

I'm currently reading a novel called "Eucalyptus". Yes, it's Australian. I've learned this much about Eucalypts: the more you learn about them, the more impossible they are to identify. The tree in front of my house may be a Silver Dollar Gum (E. polyanthemos), but who the hell knows? It would seem that the most common in California is the Blue Gum (E. globulus), used for wind breaks such as the one above. These trees are tall, but the tallest Eucalypt, which is the tallest flowering plant, is E. regnans, a.k.a. Mountain Ash -- Australia's answer to the Redwood and the Sequoia. Australia: is it surprising that a land so detached would produce such distinct trees?