154 Biology in America 



ties of western Illinois. They were noticed especially in 

 Mercer and Rock Island counties. For several years prior 

 to this invasion no abnormal numbers were seen, and their 

 coming was remarkably sudden. An eyewitness to the phe- 

 nomenon informed the writer that as he was returning to his 

 home by moonlight he heard a general rustling in the field 

 near by, and soon a vast army of rats crossed the road in 

 front of him, all going in one direction. The mass stretched 

 away as far as could be seen in the dim light. These animals 

 remained on the farms and in the villages of the surrounding 

 country, and during the winter and summer of 1904 were a 

 veritable plague. ' ' 2 



Even the humble field mouse may perform long pilgrimages. 

 "The lemings, also, a small kind of rat, are described as na- 

 tives of the mountains of Kolen, in Lapland; and once or 

 twice in a quarter of a century they appear in vast numbers, 

 advancing along the ground and 'devouring every green 

 thing.' Innumerable bands march from the Kolen, through 

 Northland and Finmark, to the Western Ocean, which they 

 immediately enter ; and after swimming about for some time, 

 perish. Other bands take their route through Swedish Lap- 

 land to the Bothnian Gulf, where they are drowned in the 

 same manner. They are followed in their journeys by bears, 

 wolves, and foxes, which prey upon them incessantly. They 

 generally move in lines, which are about three feet from each 

 other, and exactly parallel, going directly forward through 

 rivers and lakes; and when they meet with stacks of hay or 

 corn, gnawing their way through them instead of passing 

 round. These excursions usually precede a rigorous winter, 

 of which the lemings seem in some way forewarned. ' ' 3 



Reptiles and Amphibia, because of their inadequate means 

 of locomotion and their sluggish habits, are poor travellers 

 and their dispersal must be due in the main to natural in- 

 crease or to purely passive causes such as transfer of eggs 

 by currents of water or to the agency of man. 



Apart from the insects the invertebrates are for the most 

 part inactive migrants, and here too dispersal is mainly pas- 

 sive, though some animals, such as the squid or octopus, are 

 active swimmers, and probably travel considerable distances 

 "under their own steam," so to speak. 



The means of passive dispersal of animals are numerous 

 and varied. In the transport of marine animals currents 

 play the greatest part. In this way animals are carried 

 great distances at sea, distances which are limited only by 

 the animal's power of survival and by the extent of the cur- 



a Lantz, "The Brown Eat in the United States, " pp. 16-17. 

 Lyell, "Principles of Geology," llth ed., Vol. II, p. 361. 



