THE authors of the planetesimal theory have in this volume made a further important contribution to the discussion of the problems of cosmogony. In their endeavours to establish their own theory on a sound footing, much work has necessarily to be done in the way of criticising earlier theories. The classical nebular hypothesis of Laplace has already been discussed in a series of papers by Profs. Chamberlin and Moulton, and found wanting in many respects. To them in part is due the general abandonment of this hypothesis in anything like its original form by most astronomers of the present day. The present volume of papers is directed mainly against the tidal theories developed by Sir George Darwin, and more particularly against the view that at some far-distant epoch the moon separated from the earth.