CHAPTEE IV. 



THE GLACIAL EPOCH. 



Op the 222 species and subspecies of Charadriidse, we may regard 64 as Arctic in^ their 

 distribution during the breeding-season. 95 breed in the Temperate Regions, 51 in the 

 Northern Hemisphere, and 44 in the Southern, whilst in the Tropics between them 63 

 species breed. 



Of the 19 generate which these species are referable, no fewer than 14 are represented 

 in the New World as well as in the Old. Pour are confined to the Old World and only 

 one to the New. 



When we consider, further, that about half the species are shore birds, and that of the 

 other half most are steppe or prairie birds, leaving a small minority who frequent forests 

 or the banks of rivers, it does not seem unreasonable to assume that the Polar Basin was 

 the centre of their distribution. 



If we could drive them out from this centre, and isolate them in. nineteen different 

 localities further south, and keep them there long enough to differentiate them into 19 

 species, the origin of their present genera would be satisfactorily accounted for. 



The machinery required by the ornithologist to account for the present differentiation 

 of species in this family is as follows : — First, a mild climate at the North Pole, where the 

 ancestral species might increase and multiply until it became circumpolar. Second, a 

 glacial period to drive them south in every direction, and isolate them in various distant 

 regions, where there could be no interbreeding between one party and another, where the 

 conditions of life (climate, food, enemies, &c.) should vary in the different regions, and where 

 any resultant variations in the species should be rapidly seized upon by natural selection in 

 consequence of a greatly intensified struggle for existence, caused by unusual overcrowding 

 of the bird population, so that the modification of one party should proceed on different 

 lines to those of another party for a sufficient length of time to differentiate the one 

 ancestral species of the family into nineteen species, the ancestors of the present genera. 



Third, a mild period at the North Pole to allow 5 species to go back as far as the 

 Siberian steppes, leaving one species in the southern hemisphere of the New World and 

 one in the tropics of the Old World, whilst the remaining 12 out of the 19 species returned 

 to their old home, where, the tension of overpopulation being removed, they might each 



The Polar 

 basia the 

 centre of 

 distribution 

 of the Cha- 

 radriidssi 



Past changes 

 required to 

 account for 

 present dis- 

 tribution . 



