that they are doubtless millions of years, during which there has been life on the globe, then indeed we begin to comprehend how vast must have been the capital of heat with which the sun started on its career.
But now for the question, of supreme importance so far as organic life is concerned, as to the possibility of the indefinite duration of the sun as a source of radiant energy. It may indeed be urged that there is no apparent decline in the warmth of the sun and the brilliancy of the light that it diffuses. There is no reason to think from any historical evidence, or indeed from any evidence whatever, that there is the slightest measurable difference between the radiance of the sun that was shed on the inhabitants of ancient Greece and the radiance that still falls on the same classic soil. So far as our knowledge goes, the plants that now grow on the hills and plains of Greece are the same as the plants which grew on the same hills and plains two thousand years ago. It is, of course, true that the significance of the argument is affected by the circumstance that organisms by the influence of natural selection can preserve a continuous adaptation to an environment which is gradually becoming modified. The olive grows in Greece now, and a tree called by the same name grew there a couple of thousand years ago. I do not suppose that anyone is likely to doubt that the ancient olive and the modern olive are at all events so far alike that plants identical in every respect with the olive of ancient times could flourish where the modern olive now abounds. That there have been great climatic vicissitudes in times past is of course clearly shown by the records of the rocks. It is almost certain that astronomical causes have been largely concerned in
