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expressed by Professor Ward.^[1] It is that "Great types of vegetation are characteristic of great epochs of geology, and it is impossible for the types of one epoch to occur in another." For example, the presence of a dicotyledonous leaf, no matter how fragmentary, is proof positive that the stratum containing it is Mesozoic or younger. It can not possibly be older. Again, the presence of a single scar of Lepidodendron or Sigillaria, when not in redeposited strata, is just as strong evidence that they came from a Paleozoic horizon, since not a single specimen has ever been found later than the Permian.

The application of this principle is often of the greatest aid in geology, for, as frequently happens, the strata of a region have been much displaced and distorted, and it is no uncommon thing to find Paleozoic rocks occupying the positions that should seemingly, normally be taken by Cretaceous or Tertiary strata. The stratigraphy may be so exceedingly complicated as to render it quite impossible to distinguish Paleozoic from Mesozoic strata. Nor can petrology be always depended upon to supply distinguishing marks. In such cases, which are by no means purely hypothetical, a single fossil plant may serve to set at rest all possibility of dispute.

An example of this kind is furnished by the well-known case of the beds of Chardonet in France, "studied by Élie de Beaumont in 1828 and positively referred to the Mesozoic, but in which fossil plants of the genera Calamites, Sigillaria and Lepidodendron were identified by Brongniart."^[2] At that time the principle under discussion had not been recognized and Brongniart was "inclined to admit" that these genera might have occurred in the Mesozoic, although long before his death he recognized it and realized that the genera indicated beyond question a Paleozoic age.

Another important principle, bearing upon the limitations of paleobotany, is what has been called the law of homotaxis.