464 Mr. G. A. K. Marsliall on 



out alLogetlier in certain Jocalitics, while the heaps of dead 

 individuals would encourage the attack and rapid spread 

 of bacterial foes. Indeed, the advantages to be derived 

 from the removal of the surplus from an overcrowded 

 area may probably ovcrweigh those which accrue from 

 the occasional successes in colonization, and niay more 

 than the latter account for the development by natural 

 selection of the instinct to move. The massing of the 

 movincj individuals and their flight in the same direction 

 seem, on the other hand, to have arisen by selection from 

 the beneficial results conferred by spreading into less 

 crowded areas. It is difficult to imagine any other possible 

 means by which such animals as insects could overcome 

 the effects of a sudden increase too great for the restraining 

 influences of their natural enemies — eflects which in- 

 sufficiently checked for a few generations would inevitably 

 lead to the destruction of the species in the area of over- 

 production. 



We may well inquiie why it should be necessary for 

 such emigration, with a possible successful issue in colon- 

 ization, to require the services of countless individuals 

 when the impoitation of half-a-dozen rabbits or a few 

 specimens of Picris niju-v. will, for the naturalist, change 

 the face of a continent. The results of these unintentional, 

 or intentional but ill-coiisidered, experiments do indeed 

 shake the belief in the paramount necessity for crosses 

 and the dangers of in-and-iu breeding ; but the end is not 

 yet, and the teeming colonies which have arisen from such 

 small beginnings may in time vanish from the operation 

 of deep-seated causes. The varied adaptations for cross- 

 fertilization and the prevention of in-and-in breeding are 

 so evident in nature, that we are compelled to believe that 

 they meet and counteract serious dangers which sooner 

 or later would menace the very existence of the species. 

 And among other adaptations it is significant that the 

 instinct under discussion should lead to the streaming 

 of large populations, and not of small batches of individuals 

 from an area of high pressure. 



The gregarious instinct in emigration has been observed 

 in many groups of insects beside the Lepidoptera. I need 

 only mention here the hundreds of Aiainopliila Mrsida, 

 ordinarily a solitary species, found by Fabre under a large 

 fiat stone on the summit of Mont Ventoux at a height 

 of GOOO It., and the crowds of ladybirds witnessed by him 



