We are able to publish in the present issue of the MONTHLY the address given by Mr. G. K. Gilbert as retiring president of the American Association for the Advancement of Science. The problem that he discusses is one of the most pressing for scientific workers, while at the same time it is of interest to everyone, and the address is at once an important contribution to the subject and an exposition that all can understand. The mathematical physicists find that as an abode fitted for life the earth can not be allowed a history indefinitely long--not longer perhaps than 20,000,000 years--while the geologists with equally strong arguments claim a much greater antiquity. The biologists are also concerned, owing to the time taken up by the processes of evolution, and their facts and interests range them with the geologists rather than with the physicists. The man not versed in science would also prefer to assign a long history to the earth, for while he may be ready to let the ‘dead past bury its dead,’ he looks forward even to the distant future, and the shorter the past history of the earth the less the time it will continue to be habitable. We have thus a question in the solution of which all the sciences are concerned, and one possessing a dramatic interest that appeals to everyone. The unity of science is well illustrated by such a problem. It was the subject of the address of the retiring president of the Association, a geologist; it might be taken as the subject for the address of the newly elected president, a biologist and student of the processes of evolution; and it is one to which the president of the meeting, a mathematical physicist, has given special attention. * * * * *