Where did this life come from? Lord Kelvin once rashly committed himself to the notion that life might have been brought to the earth on one of those flying pieces of rock, which we have already spoken of, which are named meteorites, or on a fragment of some other planetary body that had been cast out into space. The speculation is not so wildly improbable as it has sometimes been considered to be, because recent researches have shown that it is not impossible for life to survive at the very low temperatures which a meteorite would experience on its way through space, and also that the time which a small body would occupy in travelling, let us say from Mars to the earth, would not be too great for the prolonged existence of some germ of life on the meteorite. On the other hand, there is nothing in known meteorites to suggest that they came from worlds where conditions exist suitable for life as we know it; and, moreover, even if we shut our eyes to these improbabilities, we are no nearer to a solution of the problem of where and how life began. To say that it was brought to earth on a meteorite is merely to throw back the problem another stage, for we have still to ask how life began on the meteorite and on the planet from which it came. The indirect evidence regarding the probable beginning of the era of life on the earth is also extremely difficult to examine, and we can only say that the best geological authority leans to the idea that the conditions when life would have been possible on the earth were finished long before the earth had finished growing, and that these conditions may have existed when the earth was about the size of Mars. Consequently the first beginnings of life may have existed at depths hundreds of miles below the earth's surface of to-day. The life was then, however, only of the very simplest kind; it was probably vegetable life. Probably also the first life appeared in the ocean, though it is not altogether unlikely that it may have begun, and have gone on developing, in fresh water--in those great pits which were first formed by volcanoes and which afterwards became lakes and then seas.