252 CARNEGIE INSTITUTION 



and of America on the other had its origin somewhere in central 

 Asia, having spread fan-like in both an easterly and westerly direc- 

 tion. From various paleontologic as well as biologic facts of a more 

 or less fragmentary nature, it is generally held that such invasions 

 took place during glacial times, both before and after the maximum 

 glaciation was reached. But there are clear indications that such 

 expansion from Asia to Europe and to America is going on today. 

 In a paper read before the Biological Society of Washington, one of 

 the present writers was able to demonstrate this fact with regard to a 

 limited number of species of mammals and birds in northern Russia 

 which in their westward march are nearing the Atlantic or have 

 reached it. The data for northeastern Siberia and Alaska are still 

 more fragmentary, yet there are indications pointing to a similar 

 eastward movement of the Siberian animals. It will be easily seen 

 how important it is to determine as far as possible the extent and 

 volume of this invasion. 



While it would seem from a contemplation of the cases known 

 that this expansion is made gradually by the species extending its 

 range at the periphery of its distribution, it can not be denied that 

 some animals may have spread by means of sudden irruption of 

 great numbers into hitherto unoccupied territory. As examples of 

 such explosive movements we may cite the occasional invasion of 

 Europe by great numbers of the Asiatic sand grouse, as in 1863 and 

 1888 ; similar, though more frequent irruptions of the spotted nut- 

 cracker ; and the well known wanderings of various rodents, such as 

 the lenmiings. A full study of these phenomena may have a very 

 important bearing upon the question of the proper interpretation of 

 many finds of fossils. If a future paleontologist were to find in 

 England the bones of the sand grouse in some deposit from 1863 he 

 might conclude that that species had been native of Great Britain, 

 and that consequently a climate like that of the central Asiatic 

 steppes prevailed in England in 1863. As the lemming during its 

 periodical wanderings may deposit its bones far beyond the limits of 

 its proper life zone, it is plain that the discovery of its fossil or sub- 

 fossil remains is not in itself sufficient proof of the former existence 

 of thai life zone at the locality in question. 



5. Relation of the Cave Fauna oi Europe to Existing Life. — It is 

 well known that caverns in various parts of central and southern 

 Europe contain the bones of animals of a kind verj- different from 

 those now inhabiting the region. In many instances these repre- 



