The foregoing essays on the physical influences affecting the surface features of the Earth are accompanied by one entitled The Abundance of Life. This originated amidst the overwhelming presentation of life which confronts us in the Swiss Alps.  The subject is sufficiently inspiring.  Can no fundamental reason be given for the urgency and aggressiveness of life?  Vitality is an ever-extending phenomenon.  It is plain that the great principles which have been enunciated in explanation of the origin of species do not really touch the problem.  In the essay—­which is an early one (1890)—­the explanation of the whole great matter is sought—­and as I believe found—­in the attitude of the organism towards energy external to it; an attitude which results in its evasion of the retardative and dissipatory effects which prevail in lifeless dynamic systems of all kinds.

Other Minds than Ours? attempts a solution of the vexed question of the origin of the Martian “canals.”  The essay is an abridgment of two popular lectures on the subject.  I had previously written an account of my views which carried the enquiry as far as it was in

xiii

my power to go.  This paper appeared in the “Transactions of the Royal Dublin Society, 1897.”  The theory put forward is a purely physical one, and, if justified, the view that intelligent beings exist in Mars derives no support from his visible surface features; but is, in fact, confronted with fresh difficulties.

Pleochroic Haloes is a popular exposition of an inconspicuous but very beautiful phenomenon of the rocks.  Minute darkened spheres—­a microscopic detail—­appear everywhere in certain of the rock minerals.  What are they?  The discoveries of recent radioactive research—­chiefly due to Rutherford—­give the answer.  The measurements applied to the little objects render the explanation beyond question.  They turn out to be a quite extraordinary record of radioactive energy; a record accumulated since remote geological times, and assuring us, indirectly, of the stability of the chemical elements in general since the beginning of the world.  This assurance is, without proof, often assumed in our views on the geological history of the Globe.

Skating is a discourse, with a recent addition supporting the original thesis.  It is an illustration of a common experience—­the explanation of an unimportant action involving principles the most influential considered as a part of Nature’s resources.

The address on The Latent Image deals with a subject which had been approached by various writers before the time of my essay; but, so far as I know, an explanation

xiv

based on the facts of photo-electricity had not been attempted.  Students of this subject will notice that the views expressed are similar to those subsequently put forward by Lenard and Saeland in explanation of phosphorescence.  The whole matter is of more practical importance than appears at first sight, for the photoelectric nature of the effects involved in the radiative treatment of many cruel diseases seems to be beyond doubt.