Birds are particularly suitable for a study of speciation phenomena because they are better known biologically and taxonomically than any other group of animals. A species is defined as consisting of a group of populations which replace each other geographically or ecologically and of which the neighboring ones intergrade or hybridize wherever they are in contact or which are potentially capable of doing so (with one or more of the populations) in those cases where contact is prevented by geographical or ecological barriers. Individual variation and the occurrence of exceedingly similar species obscure the taxonomic definition of spp. but actually they are very real natural units. Most species break up into well-defined geographical races of which the most isolated ones often have all the morphological characters of good species. The occurrence of many border-line cases is strong evidence that new spp. originate generally through geogr. variation.