Page:Popular Science Monthly Volume 59.djvu/294

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284
THE POPULAR SCIENCE MONTHLY.
THE EVIDENCE OF SNAILS ON CHANGES OF LAND AND SEA.
By HENRY A. PILSBRY, Sc.D.,
PHILADELPHIA ACADEMY OF NATURAL SCIENCES.

IF we wish to learn the history of any land area, we turn to its geology for a record of changes in the past. The time of its emergence from ocean, the age of its mountains and the details of its growth by successive increments of land elevated from the sea, all this we may expect to learn with reasonable accuracy, besides gaining a knowledge of the plants and animals which lived from time to time upon the coasts.

But we may push our inquiry beyond the shore, and ask. Over this expanse of sea did land once extend? Did an arm of the land reach to this island in the old time, or are the islands of that archipelago but the mountain tops of a sunken continent? To such questions geology gives no definite answer. In some cases, to be sure fjords tell their tale of sunken gorges, or soundings give evidence of a subsided coast, with river valleys and former coast-line indicated by submarine topography, as in the continuation of the Hudson River valley outward from New York Harbor, and the old shore-line, now at the hundred fathom contour. But these are exceptional cases; and the ocean bed, blanketed with modern deposits, usually gives but scant information to the geologist.

For the solution of the questions we must address ourselves to another and wholly different inquiry: the geographic distribution of living animals and plants.

To the pre-Darwinian naturalist, the relationships of animals among themselves and their distribution over the earth's surface were enigmas, quite insoluble upon the hypothesis of special creation. But the doctrine of descent, of the blood relationship of all the members of a genus and family, fills these problems with meaning. If we find that an island, such as England, has the same species of snails, earthworms, reptiles and fresh water crustacea and fishes as the neighboring continent, it becomes obvious that there has been a land connection in the past, for there is no other means by which any extensive fauna of these animals could have reached an island. If we take another island, and find that while it has different species from the mainland, yet they belong to the same genera, we must conclude that there has been actual land connection here also, though of more ancient date, across which the ancestors of these transformed species emigrated.