Foreword

In this edition the Romanes Lecture itself is reprinted without appendices, a clear distinction having been drawn therein between what is certain and what is uncertain or speculative.

The time is not yet ripe for dealing with the uncertain portions in a definite manner; while to treat them in their present stage would introduce technical difficulties and require much space; consequently it is thought better to issue the original lecture simply, as adequately representing the ascertained position to-day, and sufficiently suggesting further developments.

Fuller information can be gained from the author's treatise on 'Electrons.'

December, 1906.