Emigration 359 



day; my 13 dogs outside the tent could be heard killing them at frequent 

 intervals during the night. 



When we started for the new post, the sea ice was covered with a moving 

 mass of lemmings, all headed in an easterly direction. They stopped at 

 nothing. Untold thousands plunged over the ice into the water of a lead, 

 about a foot below, and swam the 10 or 15-foot channel, but were unable to 

 climb up the sheet ice on the other side. They perished in large numbers 

 in these leads, but here and there, they found passages up the ice and 

 blindly continued their journey. Around the ends of the leads they pressed 

 on without interruption going through the pools of water, lying on the surface 

 of the ice without deviation and without the slightest hesitation. This scene 

 extended as far as we could see in any direction. The natives later in- 

 formed me that at Kol-gyuak-a river 45 miles east of Perry-the same thing 

 was going on at the same time. I do not know how far west it extended, 

 but it was the same for at least 15 miles west of the post. This mass 

 migration lasted for about 10 days and reached its peak about May 3 or 4. 



In travelling from the old post to the new, my dogs grabbed up and ate 

 so many lemmings while they were running that their stomachs distended 

 to a noticeable degree. They were so surfeited that they were useless for 

 further work until they had gotten over their abnormal feeding. I had to 

 rest them for 24 hours to allow them to get over their gluttony. An esti- 

 mate of the average density of lemmings during this migration would be 

 one to the square yard. 



Studies of the migratory locust have shown that under ordinary 

 conditions this insect exists exclusively in a form known as the soli- 

 tary phase. Particular conditions of temperature or dryness cause a 

 great increase in the numbers of the species, and this is followed by 

 the development in the population of the swarming phase of the 

 locust, differing in color and in structure from the solitary phase. 

 With the appearance of the swarming phase, emigration of great num- 

 bers of locusts begins (Fig. 9.20). Huge swarms of locusts, such 

 as were recorded in Biblical times, spread into surrounding areas 

 and do untold damage to crops and other vegetation. Since the 

 newly invaded regions are less favorable for the maintenance of the 

 species, fewer of the young survive, and numbers are also reduced 

 by enemies that have been attracted by the plentiful food. As the 

 density of the locust population diminishes, the swarming phase dis- 

 appears and the solitary phase reappears. Gradually the locust dies 

 out of the invaded areas, and the species shrinks back to its original 

 range (Uvarov, 1931). 



In this chapter we have considered the wide variety of relations 

 among members of the same species. Other individuals of the same 

 kmd become abundant in an organism's environment as a result of 

 reproduction, of passive transport, or of active locomotion. We have 

 seen that, as numbers of a species increase, eventually harmful effects 



