542 THE POPULAR SCIENCE MONTHLY. 



be further observed, in support of our thesis, that species in- 

 capable 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 mar- 

 ten, 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 charac- 

 teristic type — the beaver — is widely diffused in Europe and in 

 Canada. The differences which the old naturalists defined be- 

 tween the European and American beavers are of the most super- 

 ficial character, while contemporary zoologists 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 propor- 

 tions 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 (Coitus 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 con- 

 nected. 



So abundant are the proofs of a communication by land be- 

 tween Europe and America during a recent age of the earth, 

 that it does not seem too presumptuous to declare it clearly cer- 

 tain. 



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 



* 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 dif- 

 ferences are of so little importance as in no way to authorize a specific distinction. 



