  We calculate the expected flux profile of comets into the planetary system
from the Oort cloud arising from Galactic tides and encounters with molecular
clouds. We find that both periodic and sporadic bombardment episodes, with
amplitudes an order of magnitude above background, occur on characteristic
timescales ~25-35 Myr.
  Bombardment episodes occurring preferentially during spiral arm crossings may
be responsible both for mass extinctions of life and the transfer of viable
microorganisms from the bombarded Earth into the disturbing nebulae. Good
agreement is found between the theoretical expectations and the age
distribution of large, well-dated terrestrial impact craters of the past 250
million years.
  A weak periodicity of ~36 Myr in the cratering record is consistent with the
Sun's recent passage through the Galactic plane, and implies a central plane
density ~0.15 M_Sun pc^(-3). This leaves little room for a significant dark
matter component in the disc.
