The cover image was created by the transcriber and is placed in the public domain.
The essays which now appear for the first time in the formof a single volume were not written upon any prearranged plan,but have been published separately at various intervals duringthe course of the last seven years. Although when writing theearlier essays I was not aware that the others would follow, thewhole series is, nevertheless, closely connected together. Thequestions which each essay seeks to explain have all arisengradually out of the subjects treated in the first. Reflecting uponthe causes which regulate the duration of life in various forms,I was drawn on to the consideration of fresh questions whichdemanded further research. These considerations and the resultsof such research form the subject-matter of all the subsequentessays.
I am here making use of the word ‘research’ in a sensesomewhat different from that in which it is generally employedin natural science; for it is commonly supposed to imply themaking of new observ