Subject to irregular changes in quantity or quality.
...they sat down beside the hearth in the adjoining room, over which the embers of the wood-fire cast a fluctuating light; now the long shadows falling duskily around—now dispersing them with bursts of brilliant flame, as the lighter wood kindled into a short-lived blaze.1834, L[etitia] E[lizabeth] L[andon], chapter X, in Francesca Carrara. […], volume II, London: Richard Bentley, […], (successor to Henry Colburn), →OCLC, page 105:
From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms.1859, Charles Darwin, On the Origin of Species: