Of course we have the problem of applying cubby-hole thinking to a spectrum world. Technically, the defintion of species involves the concept of inter-fertility. This means that if two individuals can mate and have fertile offspring, they are in the same species. Horses and donkeys can mate and have offspring, but the offspring, mules, are not fertile. Darwin's finches strain this definition in that species A and species B are inter-fertile, species B and species C are inter-fertile, species C and species D are inter-fertile, but species A and species D are not inter-fertile. Breeds are of the same species but have been bred for specific characteristics. Offspring of a great Dane and a German shepherd will have characteristics of each in varying amounts and will be completely fertile. The great Dane and the German shepherd are the same species but are different breeds.