HISTORY AND NATURAL HISTORY 33 



parently, in the face of winter cold, there is some 

 safety in numbers even among cold-blooded animals 

 that collect in hordes without any organization. 



A second plain exception to the general testimony 

 that crowding of non-social species is harmful are 

 the aggregations that form during the breeding sea- 

 son. Like the hibernating groups, these are very 

 widely distributed through the animal kingdom. 

 Breeding aggregations of worms, crustaceans, fishes, 

 frogs, snakes, birds and mammals or the midge in- 

 sects shown in Plate lb, for example, have long at- 

 tracted attention. Their numbers have been great 

 enough and conspicuous enough to stimulate re- 

 peated descriptions by naturalists. 



A third exception is found during times of migra- 

 tion, when animals frequently crowd together in 

 great hordes and execute mass migratory movements, 

 like those of many birds. 



However, breeding, hibernation and migration 

 aside, the older information indicated that up until 

 the point that social life is developed, crowding is 

 harmful. 



But there are many other instances of crowding 

 which do not fall under any of these classifications; 

 and it will be worth while to consider here the ex- 

 tent and the natural history of some of these dense 

 animal aggregations. Here, as elsewhere, there will 



