Popular Science Monthly/Volume 40/February 1892/Recent Oceanic Causeways
|RECENT OCEANIC CAUSEWAYS.|
THE object of this paper is the survey of the most remarkable changes that have taken place in the configuration of the land and the seas. My purpose is to show by an aggregation of proofs that the European and American continents were, to a certain extent, united at an epoch of only moderate geological antiquity. When we consider the extent of the Atlantic Ocean between Europe and America, as measured by the usual routes across it, we reject all thought of there ever having been a passage between the two continents in the present geological period. But the assertion of the former existence of such a communication should cause no surprise, if we regard the arctic regions of both shores of the Atlantic. In fact, if we follow a line drawn from the islands north of Scotland through the Faroe Islands to Iceland, from Iceland to Greenland, and from Greenland to Labrador through Davis Strait, which is crowded with islands and islets, we find a chain of lands interrupted only by spaces of sea of small extent, and in some places of no great depth. Subsidences of the ground and erosions have caused the isolation of lands which were united in former ages, when living Nature had assumed a character which has not ceased to exist down to our own days. A similar phenomenon produced the separation of England.
The application of natural history to physical geography and the history of the globe casts a full light upon this matter. The flora and fauna of North America are distinguished from those of Europe by essential traits. This fact contributes in a striking manner to establish the passage of a number of species from Europe to America. The demonstration appears complete when we look at the number and the character of the plants and animals inhabiting both continents. Among these are several anemones, Cruciferæ, violets, and a number of species of Stellaria of the pink family. The astragalus of the Alps thrives in Canada. Among the Rosaceæ we find a series of species of northern and Alpine Europe which occur also in North America—Spiræas, Potentillas and others. Numerous saxifrages, epilobiums, and honeysuckles are common, especially the famous Linnæa borealis. Heaths of several kinds, the rhododendron of Lapland, and primroses, have likewise found their way to America. The families of the scrophularias, the labiates, the borages, and the gentians are so represented in the New World by identical species. Among the arborescent plants, alders, willows, junipers, and the common yew exist in the cold or temperate regions of both worlds. While we refrain from dwelling on the grasses and ferns, the dissemination of which to great distances is one of the most common phenomena, we are able to cite plants which seem hardly adapted to leap over the arms of the sea, such as orchids and lilies of northern Europe, which are also common in North America.
The numerous world of insects furnishes hundreds of examples of species that have passed across from the arctic regions of Europe into America. Of the beetles, insects generally sedentary and possessed of means of locomotion so inferior that they would hardly venture to cross a sea with them, we can mention not less than three or four hundred species as common to both continents. We are particularly struck with the number of carnivorous species (Carabides), which, living on the land and hiding under stones, are disseminated very slowly. These species of carnivorous Coleoptera may be followed from the north of the European continent to Iceland, the shores of Greenland, Labrador, and Canada, It would be absurd to suppose that man has been able in his migrations to carry such a multitude of the lower creatures across the ocean. Notwithstanding the daily chances and the continual transportation of all kinds of food-products, the common chafer of Europe has not been introduced at any point in North America.
Lepidopterous insects (butterflies and moths), aided by a favorable wind, are undoubtedly sometimes carried over the sea; and it is not impossible that when they fall upon a land remote from the country of their origin they may live and propagate themselves there. These, however, are exceptional cases, while the Lepidoptera of the New World may be counted by the legion. The common vanessas of Europe abound in the northern parts of America, and the argynnes of Lapland and Iceland and the satyrs of the genus Chionobas live also in Labrador. The enumeration could be easily extended.
It is fair to suppose that investigations properly directed would enable us to recognize, in some American forms very close to the European, local varieties of the same species. It may be further observed, in support of our thesis, that species incapable of great displacements, such as the spiders of arctic countries and Alpine regions, have been observed in Greenland. We can furthermore draw valuable results from the survey of the geographical area of various vertebrates. The common marten, the common sable, and the ermine of the cold countries of Europe, have passed into North America. Specific differences between animals existing in different countries were formerly made too readily, but we are now more careful. A very characteristic type—the beaver—is widely diffused in Europe and in Canada. The differences which the old naturalists defined between the European and American beavers are of the most superficial character, while contemporary zoölogists only distinguish local varieties. Other rodents, like the Norwegian lemming and the variable hare, have followed the same ways as the preceding species, and spread themselves from one continent to the other. Finally, we must not forget the reindeer of Lapland, which also wanders in numerous troops in the coldest regions of North America.
The fresh-water fishes of North America constitute a group very characteristic of a single region of the globe. Yet this fauna is augmented by a few European species. A perch (Perca flavescens) should not apparently be separated from the river perch of Europe. The peculiarities in the number and proportions of the spines that garnish the opercle are so variable in individuals that specific distinctions can not be based upon them. The European river bull-head (Cottus gobio), which is spread through all northern Europe, lives in Greenland and North America. The European pike inhabits the fresh waters of North America, along with a distinct species peculiar to the country. Now, it is certain that no river perch or bull-head or pike ever left fresh water. These fishes could therefore have distributed themselves through the two continents only at some time when the lands scattered between the Old and New Worlds were connected.
So abundant are the proofs of a communication by land between Europe and America during a recent age of the earth, that it does not seem too presumptuous to declare it clearly certain.
If we carry ourselves back to the views which prevailed till recently concerning the isolation of America, we shall suffer a kind of surprise in observing most striking resemblances in living Nature on the two continents. The union between the continents probably existed only in the north, perhaps above the fiftieth degree of latitude. If we follow the most eastern parts of Asia, northern Japan, Siberia, and Kamchatka, which are separated from America by Bering Strait, or if we proceed from the American side through the peninsula of Alaska and the chain of the Aleutian Islands, we shall comprehend at once that only very ordinary geological changes may have been sufficient to bring about the separation of lands which had been long united. Looking toward the extreme north, we find no other separation between the Old and New Worlds than a simple arm of the sea, Bering Strait.
The study of living Nature in the arctic regions of Asia and America is very instructive. Let us begin with examining the vegetation. Some anemones and a ranunculus of Siberia are now common in North America. Another species of ranunculus is common to Japan, Kamchatka, Alaska, and northern and eastern America. While we admire the tulip tree in the parks of Europe, we recollect also that that beautiful exotic is one of the glories of the North American flora. But the tulip tree has recently been discovered in China. Then, there are the violets of Siberia and Japan, which are mingled also with the vegetation of North America; and a vine (Vitus Labrusca), now well known, reputed American, which grows in Japan and a part of eastern Asia. A maple is common to Japan and North America, as are also Spiræa betulifolia (birch-leaved spiræa) and Potentilla fragiformis of the rose family, some saxifrages, a crassula (Penthorum sedoides), various umbelliferous plants, the maritime alder, and a few orchids and lilies.
The animal world furnishes valuable evidences of our theory. Concerning insects I will cite only the facts most demonstrative of former communications. Some carnivorous beetles, the Carabs, insects remarkable for their forms and colors, wingless, and having only their legs as means of locomotion, inhabitants of eastern Siberia, are also found in the cold countries of North America. I first saw collections made in California, after I had already become familiar with the faunas of Europe, Asia, and America. I was then surprised to see in those collections European and Asiatic forms which were believed to be entirely foreign to America. A little French butterfly, also occurring in Siberia, the valley of the Amoor, and Japan, was found on the western coast of America. It appears to be unique in the color of its wings, which are beautifully green on the lower sides. The like- ness was most striking. Yet an entomologist, resting on trifling peculiarities hardly the signs of a variety, described it as a new species. It is impossible to admit this. It was then learned that the genus Parnassius, which were believed peculiar to the mountains of Europe and Asia, existed in California. The species were distinct from those of the Old World; according to the conventional expression, they were typical species. Afterward a species of the same genus was observed on the western coast of North America which was regarded as peculiar to Siberia and Mongolia. Papilio Hippocrates, a butterfly of a remarkable type, which was known in Japan, has been found in North America.
Passing to vertebrate animals, I confine myself to the mention of a small number of most characteristic types. Among the rodents we remark the marmot, Arctomys pruniosus, or sonslik of Siberia, which lives in Kamchatka, on the Alaskan Peninsula, and on the American continent. Among all the carnivorous animals of the family of the Mustelidæ, or weasels, we remark the sable of eastern Asia in Kamchatka, Alaska, and other northern parts of the American continent. A carnivorous animal of another group, the glutton, or wolverine, is found in the same regions.
In this latter part of my paper I have spoken wholly of animals and plants common to Asia and America, as in the former part I spoke only of those common to Europe and North America. But while I omit to make long enumerations of species, I insist on the fact that plants and animals are distributed in considerable numbers over the whole extent of the arctic regions in Europe, Asia, and America, having accomplished the whole circuit of that zone at an epoch when the continuity of the land made possible an indefinite dissemination to the full extent that climatic conditions were favorable.
With the present condition exactly determined, and the former condition recognized, a sure foundation is laid for the science of the future; new changes will be produced in the course of a few centuries in the configurations of the lands and the seas, and then men of science will be able to form theories of value.—Translated for The Popular Science Monthly from the Revue Scientifique.
- Anemone gratius, A. narcissiflora, and A. hepatica.
- Cardamine bellidiflora, Arabis petræa, and Draba lucana.
- M. O. Franchet, a botanist attached to the Museum of Natural History, has made, at my request, a complete examination of the plants of northern Europe which are diffused to a greater or less extent in North America.
- Blethera ardica, Nebria nivalis, Bembidium Grapei, Patrobus septentrionis, Pterostichus vitreus, P. arcticola, Amara erratica, A. interstitialis, A. brunnea, Platyrus Bogemanni, Miscodera arciica.
- Vanessa antiopa, V. Paolychlorus, V. Urtica, V. Atalanta.
- Argynnis Freya, A. Frigga.
- At my request, M. Leon Vaillant, my colleague in the Museum of Natural History, has examined all the specimens of the American perch (Perca flavescens) in the collections of this museum and compared them with the river perch of Europe. The recognized differences are of so little importance as in no way to authorize a specific distinction.
- Ranunculus cymbalaria.
- Trautvetteria palmata.
- Liriodendron tulipifera.
- Viola rostrata, V. Canadensis.
- Acer spicatum.
- Carabus Vielinghosi, C. meander, C. truncaticollis, Cychus angusticollis.
- The Thecla rubi of Europe and Asia; the California specimens were described under the name of T. dumetorum (Boisd).
- Parnassius Nomion.