if its mass were potentially vulnerable only to reductions caused by forces internal to an isolated parent galaxy—then a present-day observation of $M_{\rm gcs}$ would suffice, to at least a $\sim$25% level of accuracy globally, as a measure of $M_{\rm gcs}^
{\rm init}$. Passive evolution also implies mass conservation for the galaxy as a whole, so that a reliable value for $M_{\rm gas}^{\rm init}$ could be obtained, at any epoch, from an inventory of the total mass in gas plus unclustered stars and stellar remnants. Thus, a good [*observational estimate*]{} of the global cluster formation efficiency in this simple situation is provided by $${\hbox{$\widehat\epsilon$}}_{\rm cl}\,\equiv\,{{M_{\rm gcs}}\over{M_{\rm gas}+M_{\rm stars}}}\ ,
\label{eq:210}$$ which is expected to be nearly a time-independent quantity.

Realistically, of course, galaxies are not such pristine entities; interactions between systems may effect significant changes in both $M_{\rm gcs}$ and $(M_{\rm gas}+M_{\rm stars})$ over time, making it difficult to evaluate (or even clearly define) initial GCS and gas masses. For example, the formation history of an elliptical may include major mergers with gas-rich spiral systems, and it has been proposed that these might involve the formation of new globular clusters and substantial increases in $M_{\rm gcs}$ (e.g., Ashman & Zepf 