FACTS AND PRINCIPLES. The extinct flora of the earth, as evidenced by its fo--il re mains, was gradually evolved from simple forms low in the scale of life into forms more and more complex and higher in the scale. Each subkingdom. class, order. or other sub division of the vegetable kingdom had its own particular time of origin, and all experienced subsequent phases of evolution during which some reached a maximum and then either de clined and became extinct or else continued down to the present time. diminished in number,, and often degenerated in size: while other- main tained an upward development which has cone tinued to the present day. It is thus possible to divide geologic time a: a whole into a series of plant ages, and to designate each age by the name of the particular subdivision of the vege table kingdom which was predominant during that age. Such a time division, based upon cer tain great types of vegetation: would be as follows: (I) Age of Cryptophytes—Eozoic and Paleozoic time. (a) Age of Thallophytes .Arcluean, to and including the Upper Silurian period. (b) Age of Pteridophytes—Devonian and Carboniferous periods. (2) Age of Sperma tophytes—Alesozoic and Cenozoic time. (a ) Age of Gymnosperms—Triassie and .Turassic periods. (b) Age of _Angiosperms—Cretaceous, Tertiary, and periods.
Sueb a presentation of the facts is not, how ever, entirely satisfactory, for the reason that only the period: of maximum development of each designated type are indicated. method whith presents more accurately the several phases of development of each type and at the same time the development of the vegetable kingdom a, States and Territories. About the year 1856 paleobotany began to receive a constantly in creasing degree of attention as a result of col lections of fossil plants made by Government exploring parties and by the Northwestern Boundary Commission. Reports upon these col
a hole is by means of a diagram, in which the successively higher types are represented by a series of successively higher waves, each one having its origin later in ;:teologie time than the prceeding, one, and subsequently overlapping and ri-ing above it, the crests representing the period, of maximum development and a line tangent to the crests the development of plant life as a whole.
The general botanical character, or expression, Of any fossil flora e- sufficient to determine the era to which it belongs: the identification of a few of its principal genera will determine the period, and the identification of certain typical genera or species it ay he relied upon for the determination of the exact or approximate age of the particular strata or bed; in which they may occur. Recognition of the above facts has led to the acceptance of the broad principle, ex pressed by Ward. that "Great types of vegetation are characteristic of great epochs in geology, and it is impossible for the types of one epoch to occur in another." This principle is subjeet to modification, however. when fossil floras of widely separated localities and smaller geological sub divisions are compared. The order of sequence of types has been found to be the same every where. but evolution has at times been less rapid in one locality than in others, and hence the same types may occur, in different parts of the world, in strata which are not quite contem poraneous. To this identity of sequence, apart from contemporaneity, the term 'homotaxis' was applied by !Baxley.