Page:The Habitat of the Eurypterida.djvu/84

. Sediments which accumulated in the open marine waters at all times, subsequent at least to the Pre-Cambric, have been found to contain a rich and varied fauna in which were represented all of the larger groups of the invertebrate animal kingdom which are recognized today. One need only mention the prolific faunas of the Cambric of St. John, New Brunswick, and of British Columbia, the Trenton of New York, the Niagaran of New York and elsewhere, the Hamilton of the eastern United States, the Muschelkalk and Upper Jura of Germany, the Upper Cretacic of the middle and north of Europe, and the Eocenic of France and England. Not only is the number of individual fossils great, represented by many species, but these species are scattered through many phyla, just as at the present time the organisms in the oceans are numerous and diversified, no one class reigning to the complete exclusion of others. This does not mean that we shall find the same distribution according to phyla in the past, but we do know that it will be diverse. The vertebrates, for instance, cannot be of importance in faunas until their evolution has had time to take place, and thus they are not found represented in the rocks in abundance before the Devonic. Thus the important phylum of Pisces find no, or only rare representation in the early Palaeozoic rocks; but, on the other hand, there were the Crustacea throughout the Palaeozoic, especially the trilobites, which became extinct at the end of that period. And so one might nicely appose the phyla, or more often orders or families, which were represented in the past, but are not now, and in this way we would see that the past, though different from, was similar to the present, and that Palaeozoic seas, even the earliest ones, lacked not in life and in the diversity thereof.

The very nature of marine waters, their continuity and great extent, suggests migration and wide distribution through currents. Barriers there were, of course, both by land masses and ocean currents, streams of cold water, and so forth, but, nevertheless, we know that migration along the coasts of the continents took place as it does today and that many species or at least genera spread throughout all of the oceans, for if we did not believe in the forces of migration and dispersal we would not have laid down the laws of correlation which are universally recognized. In no way, then, can a typical marine fauna remain bottled up in one place, with none of its members escaping to adjacent waters; such a thing cannot happen today and it is not reasonable to suppose that it happened in any geological period in the past.