During 1966--1967, 27-day recurrence is a prominent feature of the transient modulation of galactic cosmic rays. Transient modulation of the nucleonic flux during 1966--1967 is compared with direct observations of two well-known classes of solar wind disturbances: shocks and shock-free density compressions. Both shocks and compressions are involved in effecting 27-day recurrent modulation; however, shocks appear to predominate. If most of these shocks result from implusive solar activity, long-lived active heliolongitudes may have a role in the recurrence phenomenon. Corotation and the recurrent susceptibility of near-earth space to the effects of implusive solar activity are suggested as a dual mechanism for the origin of recurrent cosmic ray variations.