Charles Darwin and the Silurian Survival

When Charles Darwin takes a break from a long sea voyage to look for shells on the Galapagos Islands, he finds two things that really shouldn't be there. The first is a sunbathing Doctor. The second is a fresh Silurian skull. When more Silurians are discovered by Darwin's crew - alive, this time - there is pandemonium. And when the island's volcano starts erupting, years ahead of schedule, it looks like all-out war. The Doctor has his hands full. Can he keep Charles - and his evolutionary theory - alive?