If a leaf or twig drops into shallow water with a clay or mud bottom, it ultimately sinks, and if then a film of clay or silt is brought down by freshets or what not, it will bury the leaf or twig, of course, filling in every slight depression. If then the buried object were raised so that it dried out and could be split open, we should find a perfect impression of the veins and other outward characters of the leaf etched in the clay. This is often such a perfect process that every detail of the leaf is left in the mud impression, and only the opportunity for this impression to become hardened into rock is needed for us to have a fossil. For these are merely the final hardened rock stages of a process that began as we have indicated, and the thousands of fossils that have been dug out of the earth prove how common the conditions for their formation must have been in certain periods of the world’s history.
But of the untold millions of fossils that have been made most have been destroyed, for the geologists tell us that the earth’s crust has been subjected to much upheaval. Mountain chains thrown up, inland seas formed, great river systems carved out, and tremendous periods of vulcanism or fire action have made the earth’s crust, at different periods, a mighty restless place. And these changes, so slow that often millions of years have elapsed before they were completed, have sometimes been favorable to the making of fossils and sometimes to their destruction. Darwin once wrote about the fossil record that he saw it “as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly changing language, more or less different in the successive chapters, may represent the forms of life, which are entombed in our consecutive formations, and which falsely appear to have been abruptly introduced.” And yet it is upon the evidence of this fossil record that most of our knowledge of the history of the plant kingdom is based.
The difficulty of getting any true picture of the beginnings of plant life is great, for those earliest stages of development were unquestionably water-inhabiting plants, whose tissue is mostly too soft and too easily decayed to make fossil impressions of them likely to be preserved. Yet fossil algæ have been found in rock strata so old that no fern or flowering plant had yet made its appearance. It is not too much to picture the world then as peopled only by cryptogamous plants of simple structure, living in the water, and land plants which to-day make up the bulk of our vegetation as not yet developed. Furthermore, there is in these earliest stages no trace of plants with any kind of a vascular system, such as all ferns and flowering plants possess. No stretch of our imagination can readily picture the earth as it in all probability was in that period, with no trees or vines or flowers, the land wholly bare of vegetation, and in the water, along its sterile shores, only unicellular or slightly more developed, wholly nonflowering plants. The conditions supporting such plant life existed for many millions of years, and some geologists have claimed that this period of time exceeded all the subsequent ones combined, so that algæ and some other unicellular plant types are the oldest in the world; and they still exist in enormous numbers.
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.
