A dialect continuum or dialect area was defined by Leonard Bloomfield as a range of dialects spoken across some geographical area that differ only slightly between neighboring areas, but as one travels in any direction, the differences accumulate in such a way that dialects from opposite ends of the continuum are no longer mutually intelligible. (It is analogous to a ring species in evolutionary biology.) The lines that can be drawn between areas that differ with respect to any feature of language are called isoglosses.