Interspecific hybrids are common in many tree genera. Some genera, such as Fraxinus , do not appear to hybridize readily. The extent of hybridization in many genera is not well known, especially among tropical trees. At higher levels of classification, hybridization is rare, so that we rarely see intergeneric hybrids in nature. A precise definition of species is not nearly as important as an understanding of the species concept.
Species do hybridize in nature, and the boundaries between some species are blurred.
It is still true, however, that species are often real, distinct groups, with recognizable characteristics that separate them from other species. You probably notice that this definition is very similar to the original definition of Ray. Although the species is the fundamental unit of classification, and the most important, it is not the lowest level. However, there is no consistent definition of these terms, and no codified method for their application. The most common designation is variety.