Excerpt from The Correlation of Geological Faunas: A Contribution to Devonian Paleontology
In the year 1881 I began a series of investigations for the purpose of discovering the laws which determine the association of fossils in faunal aggregates and their modifications in relation to geographical distribution and to vertical succession, in order to apply those laws as guides to the correlation and classification of geological formations. While these investigations have been in progress many other workers have joined in the search. Many statistics have been gathered, and observations have been extended over a wide field. A few important results have been attained, and the nature of the problem is now more clearly understood than at the outset. It seems, therefore, that this is a fitting time to review the progress already made, and to point out the more prominent results achieved and the paths along which future investigations may be guided with most promise of success.
When the investigations were begun it was already known that geological formations were marked by species of fossils differing greatly for each succeeding formation. In the early days of geology this difference was supposed to be due to extinction of old and the appearance of new forms for the first time with the income of each new formation. With this conception was associated the idea of sharp distinction between formations, each of which had a characteristic set of "Leitfossilien." The prevalence of this latter view dominated all the literature; and the presence, in a newly exploited section of rocks, of a species supposed to be characteristic of a given formation was assumed to be sufficient evidence of the presence of the formation in the new section. On this basis of determination it had become a fact that under the name of each formation there was catalogued a group of species collected from widely separated regions and found in different kinds of rocks, all of them being thus lumped together as the characteristic species of the formation considered.
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