A typical example of electron transfer (ET) mediated by a midway molecule is the initial ET in bacterial photosynthesis from the excited special pair (P*) to the bacteriopheophytin (BPh) mediated by the accessory chlorophyll monomer (BChl) stationed in between. It has been argued intensively whether this ET is sequential (where the state \m] of P+-BChl(-)-BPh exists as a real chemical intermediate) or superexchange (where it is passed as a quantum-mechanical virtual state). This ET is isomorphic to second-order optical processes (SOOP) where the initial state composed of an incoming photon and a matter is connected to the final state composed of an outgoing photon and a matter with phonons left excited through an intermediate state composed of a matter electronically excited without a photon. SOOP is a single process reducing to Raman scattering or an absorption-luminescence sequence in mutually opposite limits. Correspondingly this ET is also a single process, not composed of two coexisting parallel channels by superexchange and sequential ETs. It reduces to them only in mutually opposite limits determined by competition between the lifetime of an electron and the reorganization time of the medium at \m]. The rate constant of this ET can be formulated by extending the formulation for SOOP. It can describe satisfactorily the initial ET in photosynthesis observed. We can predict, moreover, that superexchange ET should begin to manifest itself in a low-temperature region when \m] is raised by several hundred cm(-1) from its native position, with its validity range extending toward higher temperatures as \m] is raised further.