Page:The Habitat of the Eurypterida.djvu/230

nite conclusion to be drawn, but I think that such a geographical development and confinement more satisfactorily accounts for the facts which are known than any others. We are not obliged to believe that Dolichopterus always lived in the rivers of Appalachia; the facts of distribution and relationship could be accounted for otherwise; but this belief requires fewer special conditions than the assumption of very early dispersal by rivers on the two continents, while a marine habitat is entirely out of the question. One of the strongest reasons for my conclusion that Dolichopterus was restricted to Appalachia lies in the evidence offered by the origin of the sediments. In the study of any problem if the lithogenesis of the formations concerned points overwhelmingly to one and only one history for those formations, then slight palaeontological incongruities should not be accepted as vitiating the history pointed by the facts of lithogenesis; the apparent incongruities can generally be turned into confirmatory bits of evidence if a broad enough knowledge and a scientifically guided imagination can be brought into play. Thus, when the nature of the outcrops, the lithological characteristics of the rocks, and, most important of all, the consideration of possible sources of supply for material, all point to the continent of Appalachia as the region whence the Normanskill, Schenectady, and Shawangunk deposits must have come, while these same considerations point just as conclusively to Atlantica for the Bertie deposits, then, if a fragment of a eurypterid in the Schenectady shales shows a faint similarity to a form in the Bertie, and if half a dozen specimens in the Bertie waterlime bear a slight or even pronounced resemblance to species in the Shawangunk, we must attempt to visualize the conditions obtaining on the North American continent during the early Palaeozoic and we must seek the most rational explanation, the one most in accord with our knowledge of the laws operating at present, to account for these seeming anomalies. And we should never forget that the geological record has revealed but a few specimens of most species of eurypterids, and that sometimes even a genus is described from a single individual, and that when a writer describes a new species he compares it with the ones already known, drawing analogies where he can; but species which may seem to be very much alike when one has, say, a single member, a carapace, or a claw, of each to compare, might, if a large quantity of perfect material were available, be discovered to be so different that kinship would be found to be entirely lacking where formerly it had been confidently pointed out.