Meadow Brown (Maniola jurtina), form alba.
Victorian butterfly collectors were notorious for filling case after case with pinned specimens of seemingly identical butterflies. In fact, what they were interested in were "aberrations" - minor variations in colour, pattern and size around a type specimen. In an era when the species concept was still new, this was understandable, but aberrations were a slippery slope. Some have genetic causes, some environmental, and most (I suspect), a mixture of the two (genetic sensitivity plus developmental environment). But those cabinets of dry butterflies have one big advantage over modern digital methods (from photography to micro-CT) - DNA.