Are you implying that cross species reproduction is always impossible?
No, but the examples of cross-species breeding that are observed imply a misclassification of the two specimens. If they can breed, then clearly they are not as divergent as the current classification implies. I believe the taxonomic order is a mess that should be reorganized firstly on the basic principle that organisms which can breed are obviously members of the same species, even if they have become drastically different through the forces of selection. If two organisms cannot breed, it is not necessarily evidence they are not members of the same species, since we have examples of animals which are definitely bred from the same stock, and have not “evolved” into a radically different organism, but have become divergent enough not to be able to produce offspring any longer.