31. During this period, which must have been very long as the number and importance of the geological beds attest, animal life developed largely in the watery elements, in like manner to vegetation in a previous period. The purer air, more conducive to respiration, permits some animal to live upon the Earth. The sea has been many times displaced, but without violent commotion. With this period disappeared in their turn those races of gigantic aquatic animals, replaced later by analogous species, less disproportionate in form, and of infinitely smaller size.