Much later than this, fossil algæ of comparatively complex structure have been found, showing by their frequency and more highly developed characters a more advanced stage in the development of the plant kingdom. So common were these various types of what we now call seaweeds, although most of them apparently lived in fresh water, and so widespread was their occurrence, that this Pre-Cambrian period has often been called the reign of algæ. As yet no other plants had been developed and none of these ancient types had invaded the land, which for millions of years more must have been entirely without vegetation. 2. THE DEVELOPMENT OF LAND PLANTS After the reign of algæ and other cryptogamous water plants, our knowledge of which is so unsatisfactory because of the incompleteness of the fossil record, there appeared the first evidences of plants that were able to live with “one foot on the land and the other in the water,” so to speak. How many transitional stages there may have been, and what relation any of these may bear to existing plants, is not known, or is, at any rate, so little understood that it is a disputed point. But somewhere about this period there did appear plants capable of living at least part of their life on the land, and possessing in their vascular system a structure of enormous advantage over their predecessors. It is pure speculation as to what this first land plant was derived from, or from what particular group of water-inhabiting plants it took some of its characteristics. Its appearance, in any case, was a dramatic event of the first importance. Not the least interesting feature of it is that the very plants of which we have indisputable evidences of being the first land plants have come down through the ages to the present day. For it is practically impossible to separate our modern representative from its ancient ancestor, despite all the tremendous changes that have been going on both in the history of the earth’s crust and in the vicissitudes of the vegetation in meeting those changes. There is the best of evidence that these first land plants were of the club moss family, which are relatives of the ferns. One of them, representing our present _Lycopodium Selago_ (Figure 107) so closely as to be practically indistinguishable, is a common type, as revealed to us in the fossil record. The present plant inhabits rich, moist, and mostly evergreen woods in the northern part of the globe, and is common in the Adirondack and White Mountains. [Illustration: FIG. 107.--CLUB MOSS (_Lycopodium Selago_) A club moss which has come down through the ages almost unchanged from the days when coal was being formed. Grows to-day in the north temperate zone, particularly in mountains.]