[90] October 1904. Since this Address was given the subject of radio-activity has assumed high importance in reference to questions connected with the evolution of the cosmos. Thus Prof. George Darwin, in view of the newly discovered properties of radium, has stated that he sees 'no reason for doubting the possibility of augmenting the estimates of solar heat, as derived from the theory of gravitation, by some such factor as ten or twenty' (_Nature_, 24th Sept., 1903). The same opinion is shared by Prof. Joly, who points out that the establishment of the observed gradient of temperature from the earth's surface inward 'may have been deferred indefinitely during the exhaustion of stores of radium and similar bodies at greater or shallower depths' (_Nature_, 1st Oct., 1903). More recently Prof. Rutherford, who has taken so leading a part in the discussion of radio-activity, has made the following statement: 'I think we may conclude that the present rate of loss of heat of the earth might have continued unchanged for long periods of time in consequence of the supply of heat from radio-active matter in the earth. It thus seems probable that the earth may have remained for very long intervals of time at a temperature not very different from that observed to-day, and that in consequence the time during which the earth has been at a temperature capable of supporting the presence of animal and vegetable life may be very much longer than the estimate made by Lord Kelvin from other data' (_Radio-activity_, Cambridge, 1904, p. 346). Thus two of the three physical arguments are impugned on new grounds, and the forecast of Prof. Darwin is shown to have been reasonable when in 1886 he said: 'Although speculations as to the future course of science are usually of little avail, yet it seems as likely that meteorology and geology will pass the word of command to cosmical physics as the converse.' [91] _The Age of the Earth_, Presidential Address to the Victoria Institute for 1897, p. 10; also in _Phil. Mag._, January 1899. [92] The geological arguments are briefly given in my Presidential Address to the British Association at the Edinburgh Meeting of 1892 (_ante_ p. 182). The biological arguments were well stated, and in some detail, by Professor Poulton in his Address to the Zoological Section of the Association at the Liverpool Meeting of 1896. [93] See an interesting and suggestive paper by Professor Le Conte on 'Critical Periods in the History of the Earth,' _Bull. Dept. Geology, University of California_, vol. i. (1895), p. 313; also one by Professor Chamberlin on 'The Ulterior Basis of Time-divisions and the Classification of Geological History,' _Journal of Geology_, vol. vi. (1898), p. 449.