The Evolution of Ellipticals, Spirals and Irregulars: Overcoming   Selection Bias
  The Hubble Deep Fields represent our best opportunity for probing galaxy evolution over a substantive look-back time. However as with any dataset the HDFs are prone to selection biases. These biases are extremely severe beyond z \~1.25 such that a meaningful interpretation of generic galaxy evolution is not possible. We can however extract well defined volume-limited samples at z < 1. The data are entirely consistent with passive/null-evolution for ellipticals, spirals and irregulars however this concluion is tempered by small number statistics. Alas stringent constraints on galaxy evolution await an order of magnitude increase in the number of HDFs. 
Simon Driver (St Andrews)