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T seems to be worth while to collect together what may be said on the subject of the duration of life on the globe viewed as a problem in physics, and this is the subject I propose to discuss in the present chapter. In the first place, it will be desirable to define a little more clearly the exact question which is to engage us, so as to avoid raising collateral inquiries on which it would not be convenient now to enter. Let it be first of all understood that I am not intending to discuss at present the question from its biological point of view, at least not more than to allude to the conceivability that there can be biological reasons for anticipating a termination to man's existence at some time or other. Why, it may be asked, should the human species expect to enjoy perennial existence, seeing that the facts of palaeontology show us that multitudes of races of animals have had their little day, and vanished? It would, at least, be necessary for man to see clear grounds for his belief before he could fancy himself entitled to an immunity from the destruction which seems to be the destiny of other species. Biological agents for the extinction of man have been suggested with plausibility. The influenza bacillus was