Page:Popular Science Monthly Volume 40.djvu/560

542 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