  It is widely believed that lenticular (S0) galaxies were initially spirals
from which the gas has been removed by interactions with hot cluster gas, or by
ram-pressure stripping of cool gas from spirals that are orbiting within rich
clusters of galaxies. However, problems with this interpretation are that: (1)
Some lenticulars, such as NGC 3115, are isolated field galaxies rather than
cluster members. (2) The distribution of flattening values of S0 galaxies in
clusters, in groups and in the field are statistically indistinguishable. This
is surprising because one might have expected most of the progenitors of field
S0 galaxies to have been flattened late-type galaxies, whereas lenticulars in
clusters are thought to have mostly been derived from bulge-dominated
early-type galaxies. (3) It should be hardest for ram-pressure to strip massive
luminous galaxies with deep potential wells. However, no statistically
significant differences are seen between the luminosity distributions of
early-type Shapley-Ames galaxies in clusters, groups and in the field. (4)
Finally, both ram-pressure stripping and evaporation by hot intra-cluster gas
would be most efficient in rich clusters. However, the small number of
available data in the Shapley-Ames sample appears to show no statistically
significant differences between the relative frequencies of dust-poor S0_1 and
dust-rich S0_3 galaxies in clusters, groups and in the field. It is tentatively
concluded that ram-pressure stripping, and heating by intra-cluster gas, may
not be the only evolutionary channels that lead to the formation of lenticular
galaxies. It is speculated that gas starvation, or gas ejection by active
nuclei, may have play a major role in the formation of a significant fraction
of all S0 galaxies.
