As fossils in the majority of instances are the remains of marine animals, it is mostly the temperature of the sea which can alone be determined in this way; and it is important to remember that, owing to the existence of heated currents, the marine climate of a given area does not necessarily imply a correspondingly warm climate in the neighbouring land.

Land-climates can only be determined by the remains of land-animals or land-plants, and these are comparatively rare as fossils.

It is also important to remember that all conclusions on this head are really based upon the present distribution of animal and vegetable life on the globe, and are therefore liable to be vitiated by the following considerations:-- a.

Most fossils are extinct, and it is not certain that the habits and requirements of any extinct animal were exactly similar to those of its nearest living relative.
b.

When we get very far back in time, we meet with groups of organisms so unlike anything we know at the present day as to render all conjectures as to climate founded upon their supposed habits more or less uncertain and unsafe.
c.