_Mountain Genesis_ first brings in the subject of the geological intervention of radioactivity. There can, I believe, be no doubt as to the influence of transforming elements upon the developments of the surface features of the Earth; and, if I am right, this source of thermal energy is mainly responsible for that local accumulation of wrinkling which we term mountain chains. The [1] Bull. Geol. Soc. America, vol. xxvi, March 1915. xii paper on _Alpine Structure_ is a reprint from "Radioactivity and Geology," which for the sake of completeness is here included. It is directed to the elucidation of a detail of mountain genesis: a detail which enters into recent theories of Alpine development. The weakness of the theory of the "horst" is manifest, however, in many of its other applications; if not, indeed, in all. 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.