fluctuating

“...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.”

“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.”