If you're considering many different types of wood for your hardwood floor, one thing to take into account is the hardness of a particular species. Below is a chart to help you make your decision.
The above results are based on the Janka Hardness Test. All hardnesses are approximate, and based on the specific species used for flooring (For instance, Bamboo normally ranges from 1400 to 1700, but the way it is used in flooring can yeild an actual hardness of around 1160).
Douglas Fir is typically used for construction grade products (such as 2X4's), and not for flooring. It is included simply as a frame of reference (ie. Brazilian Teak is over 5 times as hard as Douglas fir).