40 PRESIDENTIAL ADDRESS SECTION C. 



non-gregarious and non-migratory, and also that the former 

 creature is out of place in a well-watered, humid country. 

 Gregariousness in a humid country would increase the liability 

 of the locust to bacterial and fungoid diseases, potent checks 

 on multiplication where the air is moist, but almost inoperative 

 in arid regions ; while migration in a well-watered country is 

 quite unnecessary as a provision against starvation. In an 

 arid country with an irregular rainfall, such as the Kalahari, 

 vegetation grows rapidly, and is plentiful after good rains. But 

 most of the rains are local, and fall now in one place and now 

 in another, thus tending to make any one place a land of plenty 

 for plant-feeding insects at one time, but very liable to be a land 

 of great want in a few months. Under such conditions it 

 must be a great advantage to locusts to be able to seek pastvires 

 new. The insects must at times become very scarce indeed, 

 and the instinct for them to keep together on their migrations, 

 aside from probably affording some direct protection against 

 complete extirpation by enemies, presumably acts indirectly to 

 ensure the perpetuation of the species. If necessity to migrate 

 arose when the numbers were small and each locust went its 

 own way, comparatively few, I imagine, would meet with mates 

 when the time for breeding came. Hence it appears to me 

 that the gregarious migratory locust, capable of long-sustained 

 flight, is a very natural development in a wide expanse of arid 

 country with a desultory rainfall. 



It has been recognised by most students of locusts that 

 their migrations may be divided into two distinct classes, which, 

 for convenience, I call local flights and true migrations respec- 

 tively. Local flights are now in this direction, now in that, 

 sometimes are only for a mile or two, but often for a score or 

 more, and they seem prompted chiefly by a restless searching 

 for suitable feeding grounds and for suitable places for egg 

 deposition. True migrations are in some one general direction 

 that is pursued by immense swarms day after day and sometimes 

 week after week, and they are broken only for feeding and 

 during unfavourable weather. Various theories have been put 

 forward in explanation of these general flights, but I confess I 

 And none of them satisfactory. Lately I have come to think 

 that, despite all that has been written to the contrary, the keeping 

 to a more or less set direction may be due chiefly to prevailing 

 winds at the time favouring that direction. At times birds 

 undoubtedly tend greatly to keep swarms in motion, and it may 

 be that the fly parasites have a similar harassing effect ; but the 

 leading factor that keeps both the winged and hopping insects 

 so continuously on the move may, I venture to suggest, be 

 merely the irritating effect of their being crowded together at a 

 season when they are naturally active. 



I imagine a swarm to grow like a rolling snowball as it 

 traverses a thickly-infested region. Then that, as it passes 

 onward, it gradually loses in volume through the dropping out 



