A gene-for-gene relationship can evolve only in a discontinuous pathosystem. This is because it functions as a system of locking. A matching allo-infection is the equivalent of a lock being unlocked. With the end of the season, all matched (i.e., unlocked) host tissues disappear. With the onset of a new growing season, all discontinuous host tissue (e.g., new leaves of a deciduous tree, newly germinated annual seedlings, or newly emerged tissue of a perennial herb) is unmatched and each host individual has a vertical resistance that is functioning. This is the equivalent of re-locking. This alternation of matching and non-matching (or unlocking and re-locking) is an essential feature of any system of locking, and it is possible only in a discontinuous pathosystem. Conversely, in a continuous pathosystem just one matching allo-infection on each host individual is required for that individual to be parasitised for the rest of its life which, in the case of some evergreen trees, may endure for centuries. A gene-for-gene relationship is useless in such a pathosystem and, consequently, it will not evolve.