  Since the background fields of the string low energy action are supposed to
be the long range manifestation of a condensate of strings, the addition of
world sheet actions to the low energy effective action needs some string
theoretic explanation. In this paper we suggest that this may be understood, as
being due to string loop effects. We first present arguments using an equation
due to Tseytlin and then more rigorously in the particular case of type IIB
theory by invoking the Fischler-Susskind effect. The argument provides further
justification for ${\rm SL}(2,Z)$ duality between D-strings and
F(fundamental)-strings. In an appendix we comment on recent attempts to relate
the type IIA membrane to the 11-dimensional membrane.
