CREATION BY EVOLUTION
Jurassic rocks many of the coral reefs are isolated, occurring at places where warm currents from the south raised the temperature of the water. Most of these coral reefs have disappeared, and thus, though the development of the European Montlivaltia may have been continuous, it appears discontinuous owing to the imperfection of the geological record. In Europe it is therefore convenient to treat the separate groups as species; but in India, where a large number of corals were collected from one area of slightly undulating sea floor, the variation is continuous. The groups of corals are knots of individuals, or circuli.
The opportunities for tracing progressive evolutionary series of fossils in the field are not numerous; such series can be found only where thick deposits have been laid down continuously under the same geographical conditions, so that for a long period forms were deposited one above another and the intermediate forms were preserved in their right order. The Chalk is one of the formations to which this method can be applied. It is a soft, earthy limestone, in places a thousand feet thick, and was laid down as an almost continuous deposit of limy mud, so that the fossils are perfectly preserved and easily extracted; and owing to the many uses of chalk large exposures are available in inland quarries as well as in continuous sections in sea cliffs. Other groups of Chalk fossils show the same continuous evolution as Micraster. The process has been demonstrated in other formations, as by Hyatt and others for the ammonites, by Carruthers for a Carboniferous coral, by Schuchert and H. Walker for the brachiopods.
Opportunities for the study of contemporary variations are more common, both with fossils and with living animals or plants. Where organisms live in large numbers under similar conditions the attempt to divide species becomes practi-
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