Halliday (1961: Section 2.2):
... I have used the terms 'hierarchy', taxonomy', and 'cline' as general scale types.
A hierarchy is taken to mean a system of terms related along a single dimension which must be one involving some form of logical precedence (such as inclusion).
A taxonomy is taken to mean a special type of hierarchy, one with two additional characteristics:
(i) there is a constant relation of each term to the term immediately following it, and a constant reciprocal relation of each to that immediately preceding it; and
(ii) degree is significant, so that the place in order of each one of the terms, statable as the distance in number of steps from either end, is a defining characteristic of that term.
A cline resembles a hierarchy in that it involves relation along a single dimension; but instead of being made up of a number of discrete terms a cline is a continuum carrying potentially infinite gradation.