THE EVOLUTION OF PLANTS
clude that they have come into existence by descent from preëxisting organisms.
2. Gradual change. Either conditions have always been as they now are, or else there has been a change throughout the vast aeons of geological time. The evidence on this subject consists of fossils found in the rocks of the earth’s crust. The more recently formed rocks (such as those of the Tertiary period) contain fossil remains of plants very similar to those now living—in fact species of the same genera. But as we go down the geological scale to older and older rocks all evidence of species now living gradually disappears. Moreover, we find abundant evidence in the older rocks of the existence of forms not represented in the fossils of the more recent rocks and not found at all in the vegetation of to-day. Obviously, there has been a profound change in the vegetation of the earth. Forms appear, persist for awhile, and then die out, giving place to new forms. Moreover, the geological and biological evidence forces us to the conclusion that this change has come about gradually.
3. Evolution. By studying the comparative anatomy of forms in successive geological periods we learn that they resemble one another just as they would if they were related to one another like parents and offspring. When we contemplate this fact in the light of the principle of biogenesis the only logical conclusion we can reach is that the plants of one geological period have been derived from those of a preceding period by a process of descent with gradual modification. This is what is meant by organic evolution.
4. Hypothesis, theory, fact. When men study the phenomena of nature they get ideas suggesting explanations of what they observe. These ideas are of the nature of guesses, but they are rational guesses, which are fully warranted by the contemplation of the facts observed. Such a
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