The main divisions are practically universal, and evidence of their existence and of the character of the creatures living in them can be found all over the world; the smaller divisions, however, may often be local, or only of value in one continent. To the specialist even the smallest of them is of importance, and is a link in the chain of evidence with which he cannot dispense; but we are at present concerned only with the broad outlines of the history of the plants of these periods, so will not trouble ourselves with unnecessary details.[5] Corresponding to certain marked changes in the character of the vegetation, we find seven important divisions of geological time which we will take as our unit periods, and which are tabulated as follows:—
Cainozoic
I. Present Day.
II. Tertiary.
Mesozoic
III. Upper Cretaceous (or Chalk).
IV. The rest of the Mesozoic.
V. Newer Palæozoic, including
Permian.
Carboniferous.