The geographical distribution of animals, with a study of the relations ...

24 DISPERSAL AND MIGRATION. [part I.northward as far as Canada, where, according to Mr. Allen, morespecies breed than in the warm Southern States. Even in theextreme north, beyond the limit of forests, there are no less than60 species which breed ; in Canada about 1 60 ; while inThe extentCarolina there are only 135, and in Louisiana, 130.of the migration varies greatly, some species only going a fewdegrees north and south, while others migrate annually fromthe tropics to the extreme north of the continent ; and everygradation occurs between these extremes. Among those whichmigrate furthest are the species of Dendrceca, and otherAmericanflycatching warblers (Mniotiltidce), many of which breedon the shores of Hudson's Bay, and spend the winter in Mexicoor the West Indian islands.The great migratory movement of American birds is almostwholly confined to the east coast ; the birds of the high centralplains and of California being for the most part sedentary, oronly migrating for short distances.All the species which reachSouth America, and most of those which winter in Mexicoand Guatemala, are exclusively eas'tern species ; though a fewEocky Mountain birds range southward along the plateaux ofMexico and Guatemala, but probably not as regular annualmigrants.In America as in Europe birds appear in spring with greatregularity, while the time of the autumnal return is less constant.More curious is the fact, also observed in both hemispheres,that they do not all return by the same route followed ingoing northwards, some species being constant visitors to certainlocalities in spring but not in autumn, others in autumn but notin spring.Some interesting cases have been observed in America of agradual alteration in the extent of the migration of certain birds.A Mexican swallow (Hirundo klnifrons) first appeared in Ohioin 1815. Year by year it increased the extent of its range tillby 1845 it had reached Maine and Canada; and it is now quotedby American writers as extending its annual migrations toHudson's Bay. An American wren (Troglodytes ludovicianus)is another bird which has spread considerably northwards since