Before revision:  When the forms evolve spontaneously, they either grow or decay monotonically (2, 3)  . In the case of the Spanish past subjunctive, the  evolution of its two competing forms (ended in -ra and -se) was perturbed by the appearance of the Royal Spanish Academy in 1713, which enforced the spelling of both forms as perfectly interchangeable variants (4)  at a moment in which the -ra form was dominant (5).
After revision:  When the forms evolve spontaneously, they either grow or decay monotonically (2, 3) . In the case of the Spanish past subjunctive, the spontaneous evolution of its two competing forms (ended in -ra and -se) was perturbed by the appearance of the Royal Spanish Academy in 1713, which enforced the spelling of both forms as perfectly interchangeable variants (4) at a moment in which the -ra form was dominant (5).