  Cosmological models with cosmic string and texture seeded universes predict a
present abundance of very dense clumps of Cold Dark Matter particles. Their
crossing through the solar system would induce a non-negligible amount of
radiation damage to all living tissue; the severity of such an episode is
assessed. The estimated frequency of these crossings agrees with the apparent
periodicity of the paleontological record of biological extinctions. (Phys.
Lett. B, in press)
