Page:Popular Science Monthly Volume 67.djvu/472

466 which have been found in the Upper Jurassic of France. When we say that sequoias flourished in the Upper Jurassic we have a dim idea that they are a pretty old type and that, although compared to the most ancient known rocks the Jurassic rocks are mere infants, still the Jurassic age came to a close several million years ago. But we can form no more of a concept of the duration of several million years than

we do of astronomical distances, and it is only by glancing at the progress of life on the globe during all those years that we can get any sort of an idea of the remoteness of the period.

Fig. 4 is designed to show this progress of life in a general way, in the animal kingdom, which has been chosen rather than the vegetable kingdom because the changes in the former are more striking