But it often happens that neither the strata which should come above nor those that lie below can be seen. Then our second rule comes in: We can determine the age of the rock in question by its fossils. The reason of this has perhaps already been guessed by the reader. It is that as different kinds of plants and animals have prevailed at different periods of the world's history, so there have been "styles," or fashions, in creation, as well as in art. At one geological period certain curious types of fishes flourished which are now almost extinct, only a few old-fashioned survivals being found in one or two out-of-the-way places. At another period certain types of reptiles flourished vigorously, and were the leaders in their day; but they have altogether vanished and become extinct. So one type after another has appeared on the scene, played its humble part in the great drama of life; and then--"exit!" another takes its place. In the oldest and lowest of the series of rocks we find no certain trace of life at all. In the next series we find only lowly creatures, such as shell-fish, corals, and crab-like animals that have no backbone. In a higher group of rocks fishes appear for the first time. Later on, we come across the remains of amphibious creatures for the first time. Then follows (after a long unrecorded interval) an era when reptiles and birds existed in great numbers. After another long interval we come to strata containing many and diverse remains of mammals or quadrupeds. So we have an "Age of Fishes," an "Age of Reptiles," and an "Age of Mammals." Some tribes of these creatures died out, but others lived on to the present day. Thus we see that there has been a continuous progress in life as the world grew older, for higher types kept coming in. To the geologist fossils are of the greatest possible use, since they help him to determine the age of a particular set of strata, for certain kinds of fossils belong to certain rocks, and to them only. But the classification of the stratified rocks has been carried farther than this. Practical geologists, working in the field, use fossils as their chief guide in working out the subdivisions of a group of rocks, for certain genera and species of old plants and animals are found to belong to certain small groups of strata. In this way a definite order of succession has been established once for all; and, except in the case of inverted strata already alluded to, this order is invariably found to hold good.