Early Christmas morning, London time, a 70-pound British spacecraft, launched last June aboard a Russian rocket and hitchhiking behind a sophisticated European Space Agency orbital vehicle, is set to touch down on the surface of Mars. The lander, named the Beagle 2 — in honor of the sailing ship that transported Charles Darwin on his historic voyage to the Galapagos Islands in 1831 — represents an ingenious but daring entry in what is becoming a race to explore the red planet and establish once and for all whether it ever has harbored living organisms. If it succeeds, it could show up its bigger and much more expensive American cousins.