The cause of changes in Climate during past Geological periods having lately become, and being likely to continue, a prominent topic of interest, I have attempted here to bring together the more prominent features in the controversy, under the idea that to many readers, who have not either the time or the inclination to more closely examine the subject, this may be acceptable. In so doing I have, with one partial exception, confined myself to discussing the difficulties which beset all the various theories offered as a solution, rather than attempted the advocacy of any one of those theories in particular.