|BIOLOGIC PRINCIPLES OF PALEOGEOGRAPHY|
C. S. GEOLOGICAL SURVEY
CONSIDERING the breadth and intricacy of the subject assigned me, and the limited time that can be given to its consideration. it has seemed best to me to restrict my remarks to two or three of the obviously more important phases of the problem.
Aside from the study of the rock-masses themselves—which are often difficult of interpretation—reliance for an interpretation of paleogeography must be placed in the former life found entombed, and of the two biologic elements, plants undoubtedly hold a very high—probably the highest—place.
In making use of plants in the study of paleogeography we may first consider distribution. If we find two fossil floras identical or similar in all essential or important details, we feel justified in regarding them for all practical geologic purposes as contemporaneous. In order that we may be certain that the two floras are identical, they must be composed of types that are readily identifiable, that is, forms so well characterized that they may be easily and certainly recognized. As examples of such floral elements mention may be made of many ferns and fern allies, most cycads, conifers and peculiar, well-marked or characteristic dicotyledons. Having settled the contemporaneity of the floras, inquiry may next be made as to the probable manner in which the separated or isolated areas were reached by these floras. Here again we must carefully consider the character of the flora and the means for its natural dispersal. The living flora, and for that matter probably the floras from at least the beginning of the Tertiary progressively to the present time, has developed in many ways means for the comparatively rapid and wide-spread dissemination of their reproductive parts (seeds, etc.). For example, a large percentage of the members of the dominant living family of seed-plants—the Compositæ—have developed seeds with an attachment of soft, fluffy hairs which serve to float them in the air, often to great distances. In many other living groups there are similar, or at least as effective, devices for dissemination, but as we go back in time adaptations calculated to be of aid in distribution grow less and less, and soon even seeds of any kind are unknown, or known but imperfectly, and reproduction is normally by means of spores, that is, reproductive bodies in which there is no embryo already formed when they leave the parent plant. It is obvious that plants that are reproduced by seeds, in which there is both an embryo and a supply of food for use during germination, must possess a decided advantage over those reproduced by means of spores.