Thus, if λ max 1, then the approach to x * will occur (approximately) along the c 2 -nullcline in the phase-plane. However, if λ max is order unity, then the trajectory will move (although not initially along the c 2 -nullcline) until it catches x * , at which time the indicator reaction will remain in a QSS. Thus, if λ max is large enough, a transient window occurs before QSS can be imposed, which is interpreted (geometrically) as the approach to x * in the phase-plane. In either case, we have an inner solution that approximates the approach to x * , and an outer solution that closely follows x * as it rolls back to the origin: