It is important to have a general knowledge of the present distribution of plant families, and especially of the causes which determine the several botanical regions, before venturing to apply our knowledge of plant geography in the past to the question of geologic climates. The expression “geologic climates” is a convenient one to use in the present discussion: Whitney remarks “it is a term under which are commonly classed those real or supposed variations of the earth's climatic conditions, which, having taken place in past ages, before the historic period, can only be proved to have occurred by means of geological evidence.” The present facts of plant distribution cannot be explained, as botanists used to suppose in the earlier period of plant geography, by regarding special floras to have been determined by climatal conditions. Climate undoubtedly plays a most important part in setting the limits to the distribution of different plants; but we have also to take into account the fact that the distribution of the present is the result of the cooperation of a number of different agents, which have been at work in past geological periods and whose actions have been to a large extent dependent on changing geographical conditions. There are several agents whose share in determining distribution is by no means unimportant. We may probably assume that the physical agents which govern the distribution of plants in our own time have acted during the several geological periods.