This subject is so vast, and yet so imperfectly studied; it offers so many striking illustrations of mutual-aid habits, subsidiary to the main fact of migration--each of which would, however, require a special study--that I must refrain from entering here into more details. I can only cursorily refer to the numerous and animated gatherings of birds which take place, always on the same spot, before they begin their long journeys north or south, as also those which one sees in the north, after the birds have arrived at their breeding-places on the Yenisei or in the northern counties of England. For many days in succession--sometimes one month--they will come together every morning for one hour, before flying in search of food--perhaps discussing the spot where they are going to build their nests.(8) And if, during the migration, their columns are overtaken by a storm, birds of the most different species will be brought together by common misfortune. The birds which are not exactly migratory, but slowly move northwards and southwards with the seasons, also perform these peregrinations in flocks. So far from migrating isolately, in order to secure for each separate individual the advantages of better food or shelter which are to be found in another district--they always wait for each other, and gather in flocks, before they move north or south, in accordance with the season.(9) Going now over to mammals, the first thing which strikes us is the overwhelming numerical predominance of social species over those few carnivores which do not associate. The plateaus, the Alpine tracts, and the Steppes of the Old and New World are stocked with herds of deer, antelopes, gazelles, fallow deer, buffaloes, wild goats and sheep, all of which are sociable animals. When the Europeans came to settle in America, they found it so densely peopled with buffaloes, that pioneers had to stop their advance when a column of migrating buffaloes came to cross the route they followed; the march past of the dense column lasting sometimes for two and three days. And when the Russians took possession of Siberia they found it so densely peopled with deer, antelopes, squirrels, and other sociable animals, that the very conquest of Siberia was nothing but a hunting expedition which lasted for two hundred years; while the grass plains of Eastern Africa are still covered with herds composed of zebra, the hartebeest, and other antelopes.