Popular Science Monthly/Volume 59/July 1901/The Evidence of Snails on Changes of Land and Sea
|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.
It is obvious that animals with feeble powers of travel are of the greatest value in these researches, because they indicate more ancient and more enduring changes of sea and land than freely mobile creatures, such as birds and quadrupeeds, which may spread, conditions favoring, with great rapidity. Moreover, the invertebrates have changed much more slowly than higher animals; their evolution has been slow. Almost the whole great drama of mammalian evolution has been acted during Tertiary time, while there has scarcely been generic change in the mollusks! Mammals and birds reflect in their distribution the later earth movements, the invertebrates and fishes the earlier.
The Helix snails are particularly well adapted to show ancient faunal relationships, as they occur under one or another form in almost all lands. But it is only in the present decade that their anatomy has been understood, and the true relationships of the various groups of the family recognized. One of the most interesting developments of the anatomical study of land snails has been the demonstration of a close relationship existing between Helices of the Philippine Islands and eastern Asia and those of California, Mexico and the Greater Antilles.
Years ago Dr. Karl, Semper, in his Travels in the Philippine Archipelago, showed that the arboreal Helices of the Philippines are provided with a muscular sack containing a calcareous needle—the so called ’dart'—and surmounted by a mucous gland, the whole being
an appendage of the reproductive organs. In species of both China and Japan the same peculiarities are found in these organs. It was already known that the Helices of Europe have a similar sack and dart, but the associated mucous gland is split into finger-like tubes and removed from the sack to an adjacent duct. The function of the dart apparatus is not well understood. The snails thrust their darts into one another during the mating time, and hence the dart is believed to be an excitation organ.
Now, when the Helices of California, Mexico and a part of those of the West Indies were examined, it was found that they have the dart apparatus, agreeing with species of Japan, China and the Philippines, not with those of Europe or of eastern North America; for, to emphasize this resemblance, the Helices of the middle and eastern United States are anatomically totally unlike the Californian, Mexican and Antillean, having no dart-sack or mucous gland.
We are, therefore, confronted with a group of snails inhabiting both borders of the greatest ocean, but agreeing so closely in anatomy that no hypothesis but that of a common origin, descent from common ancestors, is conceivable. Our American dart-bearing Helices must surely look to distant shores in far-off times for their ancestry.
Map showing distribution of Dart-bearing Helices in Vertical Lines. Cretaceous Sea in Broken Horizontal Lines. Being on Mercator's Projection, the Northern Ranges of the Dart-bearers in America and Asia appear much more separated than they really are.
We know, however, that in Upper Cretaceous times the Arctic lands were not, as now, clad in the scanty green of mosses, lichens and herbs, with few deciduous trees, except stunted willows and the like, but they bore noble forests of magnolia, beech and birch, with red-woods and pines—such forests as snails love and thrive in, doubtless with abundance of sheltering windfalls and rotting boles to give them refuge, and decaying leaves to feed the fungi and tender herbage, which are the food of snails.
It is not unlikely, then, that, in the distant past, when a kinder climate allowed the forests of the temperate zone to extend to the Arctic shores in Alaska and Siberia, the snails went with them; and, if we assume a very moderate elevation in the region of Bering Strait, connecting Alaska and Asia by a land bridge, there would be no bar whatever to the spread of forest trees and the emigration of snails from one continent to the other.
The distribution southward of the snails and other inhabitants of the forest would be merely a question of time in the absence of barriers in the form of lofty mountains, deserts or arms of the sea, running across their path.
There are good reasons for believing that the dart-bearing snails originated in the Orient, and, if so, their migration was eastward to America. In all probability, the slowly upbuilding land mass in western America had none of the higher land snails before the advent of the Asiatic snails in the later Cretaceous, as it was profoundly isolated in earlier mesozoic and preceding time, so far as existing geological data show. On reaching America, the snails spread southward. Why, then, it may be asked, do we not have the descendants of the Asiatic dart-bearers in eastern North America? There are several reasons. During the Cretaceous period an inland sea extended from the Gulf of Mexico, through the Dakotas and northward to the Arctic Ocean, in the neighborhood of the Mackenzie River. This would prevent the eastward spread of the snail emigrants from Asia. Since that time, the increasing height of the Rocky Mountains, and the arid conditions of much of the mountain region, with its poverty in deciduous trees, would be, and is to-day, an effectual bar to the eastward distribution of the Pacific slope snails.
In the Far West, however, no barriers prevented the southward spread of the dart-bearing Helices. They pushed south to Mexico, and, perhaps later, to the Andean region of South America. Ther was also undoubtedly a land bridge connecting an Antillean continent or archipelago with Central America, over which the dart-bearers passed to the Antilles. This connection is shown by many other groups of land snails also, the distribution of which can be explained in no other manner.
The Helices of the West Indies lend no aid to those who advocate the hypothesis of an 'Atlantis' bridging or partially bridging the Atlantic, for they are not allied to species of Madeira, the Azores, Cape Verde or Canary Islands. Their anatomy is vastly nearer Mexican, Californian and East Asiatic groups.
No generalization based upon the distribution of snails, or of any one group of animals, can be satisfactory unless it is supported by the evidence of animals of other groups, and by that of plants. In extending the data relative to the zoogeography of America and Asia, it may be said that the evidence of the naked snails or slugs fully supports that of the Helices. The West American slugs have their cousins in China and the Himalayas, not in eastern North America. The freshwater crayfish of our Pacific slope belong to the Old World genus Astacus, not to the East American genus Cambarus. The evidence of fishes seems to strongly favor a former connection of Asia and America. Thus. Gunther deduces a Central Asian origin for the
Cyprinoids, or Carp family, which is also very numerously represented in America. Moreover, he regards the Chinese species of Catostomus, or 'sucker,' as a return emigrant from America to Asia. The North American catfishes belong to an East Asian group of the family; and our garpike has a representative in Chinese waters.
Such evidence as the higher vertebrates afford do not strengthen the case stated for the snails, because their evolution has been vastly more recent and rapid, and their means of distribution are far less restricted. Thus the horses have attained their present distribution since the Pliocene, but they are capable of spreading rapidly wherever pasturage is to be found. The wide range of such groups as this, and the birds, is limited only by markedly unfavorable physical conditions, and their presence in both the Old and New Worlds merely indicates that up to quite recent times there has been a land bridge over Bering Strait.
The identical species of plants in Japan and the United States, elaborately discussed by Dr. Asa Gray, are also, in many cases, it would seem, comparatively recent emigrants into one continent or the other, not old enough to have become changed by new surroundings; or they are plants which lived in the Tertiary forests of Greenland and British America, which Heer and others have made known. Through stress of climate, this circumboreal flora has been driven southward, many of its species to be changed by the vicissitudes of the march, while others still flourish unchanged in the two continents.
- Dawson maps the Cretaceous inland sea as extending to the Arctic Ocean. During the earlier Cretaceous it also reached the Pacific, though an archipelago probably extended north to Alaska; but in the Laramie there was a broad belt of land to the westward of the Cretaceous sea or lakes. See map, which represents the probable extent of the sea at the beginning of the Laramie.
- The Study of Fishes, p. 244.