  We have evaluated a systematic effect on counts-in-cells analysis of deep,
wide-field galaxy catalogues induced by the evolution of clustering within the
survey volume. A multiplicative correction factor is explicitly presented,
which can be applied after the higher order correlation functions have been
extracted in the usual way, without taking into account the evolution. The
general theory of this effect combined with the ansatz describing the
non-linear evolution of clustering in simulations enables us to estimate the
magnitude of the correction factor in different cosmologies. In a series of
numerical calculations assuming an array of cold dark matter models, it is
found that, as long as galaxies are unbiased tracers of underlying density
field, the effect is relatively small ($ \simeq 10%$) for the shallow surveys
($z <0.2$), while it becomes significant (order of unity) in deep surveys ($z
\sim1$). Depending on the scales of interest, the required correction is
comparable to or smaller than the expected errors of on-going wide-field galaxy
surveys such as the SDSS and 2dF.
  Therefore at present, the effect has to be taken into account for high
precision measurements at very small scales only, while in future deep surveys
it amounts to a significant correction.
