Nor was the earth peopled wholly by this ancestor of _Lycopodium Selago_, for we find at this time, or just after it, a great development of plants of this type. Some of these were giant, treelike club-mosses that have been so well preserved as fossils that even their internal structure and spore-bearing characters are well known. Many other strange relatives of our modern club mosses flourished in those days, some of which have wholly disappeared, as have all the treelike forms. These highly organized club mosses, quite unlike any modern representative of the family, appear to have been crowded off the earth by other and subsequent types of vegetation, while _Lycopodium Selago_, and about thirty related species, have persisted to the present day; not precisely in all cases as they were in this dawn of a land flora, but in many cases with modern structure and reproductive processes so close to the ancestral types as to be nearly identical. Perhaps nothing gives one a better impression of the tremendous time that must have elapsed before the appearance of these ancient club mosses than the very slight modifications from their ancient condition which their structure at the present time exhibits. While nothing is certainly known of their origin, when they first appeared they were plants with a well-developed vascular system, having stems and leaves quite unlike any of their predecessors’, and a reproductive process almost precisely like their modern descendants. In other words, if they have changed so slightly in all the millions of years since our rock-written records of them first occur, what an infinitely greater period must have elapsed down the dim vista of the ages before their appearance. Of this period, with the exception of fossil algæ, we know practically nothing, and, worst of all, the actual transition from a wholly water-inhabiting flora to these certainly land-inhabiting club mosses may never be known. For, added to the difficulty of water plants being preserved as fossils, already mentioned, is the fact that as they are the oldest, they are found in the deepest strata and, consequently, the hardest to find; and due to changes in the earth’s crust, these ancient fossil-bearing strata have often been much disturbed. The conclusion appears to be indicated that the origin of a land flora came about with the appearance of these ancient club mosses, which are not mosses in our present-day interpretation of those plants, and that at about the same period many other plants also were found, the whole vegetation resembling nothing that exists at the present time, but many of the different kinds of this ancient flora showed unmistakable evidence of being the progenitors of many plants that exist to-day. What these were, and particularly what they accomplished, both in the history of the plant kingdom and in making the world habitable for man who did not come for millions of years after they were preparing the way, will be considered in