  After commenting on the early search for a mechanism explaining the Newtonian
action-at-a-distance gravitational law we review non-Newtonian effects
occurring in certain ansatzes for shielding, screening and absorption effects
in pre-relativistic theories of gravity. Mainly under the aspect of absorption
and suppression (or amplification), we then consider some implications of these
ansatzes for relativistic theories of gravity and discuss successes and
problems in establishing a general framework for a comparison of alternative
relativistic theories of gravity. We examine relativistic representatives of
theories with absorption and suppression (or amplification) effects, such as
fourth-order theories, tetrad theories and the Einstein-Cartan-Kibble-Sciama
theory.
