Before proceeding to describe the plants of Paleozoic and later time, the reader should be impressed with the important fact that plants of higher and higher types came into existence throughout geological time in almost exactly the botanical order of their classification, that is to say, from the very simplest types (thallophytes) of Proterozoic time there were gradually evolved, through the long geological ages, higher and higher plant forms reaching a climax in the complex and highly organized plants of the present time. This is the most significant general fact in regard to the geological history of plants. For the convenience of the reader the largest subdivisions in the classification of plants are here given. OUTLINE CLASSIFICATION OF PLANTS I. Cryptogams {1. Thallophytes (e.g., seaweeds, mushrooms) (seedless and {2. Bryophytes (e.g., mosses) flowerless) {3. Pteridophytes (e.g., "club mosses," "horsetails," ferns) II. Pteridosperms { (seed-bearing, { (e.g., seed ferns--wholly extinct) flowerless) { III. Phanerogams {1. Gymnosperms (e.g., cycads, conifers) (seed-bearing, {2. Angiosperms (e.g., grasses, lilies, flowering) { oaks, roses) Throughout the first two periods--Cambrian and Ordovician--of the Paleozoic era, plant life appears to have made little or no progress toward higher forms. The very simple Thallophytes (e.g., seaweeds) continued to secrete concentric layers of carbonate of lime in almost exactly the same way as during the middle and late Proterozoic era. Remarkable reefs of such forms occur in the late Cambrian limestone near Saratoga Springs, New York, where one locality has been set aside as a state park. During the Ordovician there were seaweeds of the more familiar branching types without carbonate of lime supports, and these have left very perfect impressions in some of the Ordovician strata. During the Silurian period seaweeds continued, as, in fact, they did throughout succeeding geologic time to the present. The Silurian strata seem to contain some vestiges of the first-known land plants, though the records are meager and some of the specimens are of a doubtful character. Most interesting of all is a fern or fernlike plant found in France. When we consider the profusion of land plants (all of relatively simple types) of the next or Devonian period, it seems certain that their progenitors must have existed in the Silurian, and their remains may very likely be discovered.