  Growing evidence has been accumulated for that some gas-rich dwarf galaxies
are formed from material liberated by galaxy collisions and/or mergers. Also,
gas-poor dwarf elliptical galaxies are often found in the central regions of
clusters of galaxies. These observations suggest strongly that the formation of
most dwarf galaxies is linked to galaxy interactions. Therefore, now seems like
the right time to investigate the formation efficiency of such tidal dwarf
galaxies. Adopting the galaxy interaction scenario proposed by Silk & Norman,
we find that if only a few dwarf galaxies are formed in each galaxy collision,
we are able to explain the observed morphology-density relations for both dwarf
and giant galaxies in the field, groups of galaxies, and clusters of galaxies.
It seems worthwhile noting that the tidal dwarf formation may be coupled with
the transformation from gas-rich disk galaxies to early-type galaxies such as
S0 and elliptical galaxies.
