 The empirical sign of this essential unity of the species over time is the ability of its members to interbreed and produce fertile offspring, a criterion that takes precedence over similarities of anatomy or habits of life. The horse and ass must be two different species because they cannot interbreed and produce fertile offspring, whatever may be their anatomical resemblances. The dogs, on the other hand, must, in spite of great morphological differences between breeds, constitute one species because of their interfertility.