Page:Popular Science Monthly Volume 51.djvu/680

664 plants before they learn the descriptive terms applied to them. To have a knowledge merely of the names of plants is to possess a series of jug-handles without the jugs or anything in them. The handles belong with the jugs, and jugs should have, at times at least, something in them. Now, the truly scientific man who seems to be devoting his time to acquiring a knowledge of the names of plants, is in reality learning much more about them. He sees that plants of different sorts resemble one another in greater and less degrees. Those which resemble one another, not merely in superficial characters, which may be due merely to like conditions of growth and life, but in the more fundamental and less obvious characters, he assumes are related; and his studies lead him to formulate a classification which shall indicate in what ways and in what degrees the different plants are related. So we see the first step in the development of botany, the study of plants with a view to arranging them in some classification which shall subserve the convenience of students and at the same time indicate the relationships of plants to one another. But the word relationship implies something more than superficial resemblance. The resemblance is an index of descent. The older botanists—Jussieu, Linnæus, and others like them—believed that plants are now as they were created created either at the beginning of the world or brought into existence later by separate acts of creation. The more critical observations of later years have shown that no two plants are exactly alike, that the offspring are not the duplicates of the parents, that plants are constantly changing as organisms and as generations of organisms. Certain influences cause certain changes; the different conditions to which plants are exposed in sunny and shaded, in moist and dry, in exposed and sheltered positions—the climatic, the geological, the geographical conditions—all have their effects. And so the study of plants extends from the examination of those just about us to a comparison of these familiar forms with those in other localities. There develops the science of geographical botany, which seeks to penetrate the reasons for the existence of certain plants as characterizing the North American, the central European, the Australian, and other floras.

In order to solve the problems thus encountered, the botanist must know not merely the present geographical and geological conditions of our globe, or of that part of it which he especially studies, but also what its geological history has been. This throws light upon many questions. For example, the North American flora is much richer than that of Europe. We gain some idea as to the reasons for this when we realize that the last ice period made much of Europe and of North America uninhabitable, as well for plants as for animals. All were compelled to migrate