THE RECORD OF THE ROCKS
Almost everywhere there are breaks, due either to some cessation of the supply of rock-material or to the washing away by sudden local currents of rock already deposited. Or again, though rock may have been laid down without pause, the conditions may have changed, so that, for example, sandstone is succeeded by fine-grained clay or shale, and this again by limestone. With the change of conditions there was a change in the character of the animal and plant communities living on the sea bottom or in the waters above. Some creatures live at one depth, some at another; one kind prefers a sandy bottom, another prefers a limy ooze, and so on. Consequently, where a great thickness of rock consists of layers differing in composition, it is not possible to trace a single race of animals up through the whole sequence in any limited region. The race migrated and its descendants must be sought elsewhere.
One of the best examples of a rock that was laid down continuously through a long period of time and yet retained its general character is the Chalk of southern England and northern France. This soft limestone was laid down in a relatively shallow and apparently calm sea, and it contains many fossils of marine animals. Among these fossils sea-urchins are very common, and a careful study has been made of one particular genus of heart-shaped sea-urchin named Micraster. A. W. Rowe, a physician of Margate, devoted his holidays to collecting these heart-urchins from the Chalk of England, foot by foot. He was able to show that what appeared to be a distinct species found at the bottom of the Chalk gradually changed into a different species found at the top. This change is almost imperceptible, but it can be traced in every part of the fossil shell, and it takes place in the same way in all parts of the country. Here is an example of evolution caught in the act. If we were to take a set of
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