150 UNSOLVED PROBLEMS. 



the gravest errors in relation to their own lives and safety 

 biting reindeer, for instance, and getting trod to death by 

 the pain- and panic-struck animals ; entering rivers or lakes 

 unnecessarily, and so being drowned in swarms ; and finally 

 losing their way apparently in the sea. As in the case of 

 the butterflies, none ever return, and the exact locality 

 whence they come is equally unknown. In the lemming 

 migrations we have again the use of a bee line a straight 

 march apparently towards the sea, a march that allows no 

 obstacles to stand in the way. But while in many cases of 

 bee lines there is or may be some knowledge of the ground 

 to be traversed, of the point of departure and of the goal or 

 home, here there appears to be absolute topographical ignor- 

 ance associated with other singular and no doubt morbid 

 forms of stupidity. 



Obviously the mode of way-finding is not the only puzzle 

 among the phenomena of migration, nor is it perhaps the 

 most interesting of several problems that suggest themselves 

 for solution an connection with these phenomena. Thus of 

 the causes of migration itself, on the one hand, and of certain 

 of its results on the other, we have at present no satisfactory 

 knowledge. As regards bird migrations, it has been sug- 

 gested that their cause is or may be 



1. Search for food or water, or for 



2. Breeding ground, or for 



3. Light or warmth, or both conjoined. 



No doubt some or all of these causes or motives may be 

 operative in certain cases, as in autumnal southward migra- 

 tions; but such suppositions do not account for all the 

 phenomena for instance, for the periodicity, apparent sud- 

 denness, and imperiousness of the impulse in the swallow, 

 salmon, or lemming. There are migrations on the small 

 scale the reason of which is obvious and intelligible for 

 instance, the daily or regular migration of a hen and her 

 chickens in search of food, or the less regular but equally 

 familiar migration of the reindeer to avoid insect pests, such 

 as the gadfly. But it does not follow that migrations on a 

 larger scale differ from these migrations of the hen, reindeer, 



