  We have carried out two extremely deep surveys with SPIRE, one of the two
cameras on Herschel, at 250 microns, close to the peak of the far-infrared
background. We have used the results to investigate the evolution of the
rest-frame 250-micron luminosity function out to z=2. We find evidence for
strong evolution out to a redshift of around 1 but evidence for at most weak
evolution beyond this redshift. Our results suggest that a significant part of
the stars and metals in the Universe today were formed at z<1.4 in spiral
galaxies.
