There exists a more or less continuous chain of sea-gulls which starts in England and which goes all around the globe just to return close to the location where the chain began. When we go along the chain the gulls gradually change. At the one end we find black-backed gulls, at the other herring gulls. The gulls of these two types cannot form a breeding population (they cannot interbreed). Thus on the basis of an interbreeding-criterion they belong to different species. But everywhere in whatever section in the chain the gulls are able to interbreed with gulls from neighboring sections, so every set of gulls belongs, according to that same criterion, to the same species as its neighbors (COHEN & STEWART, 1995, The Collapse of Chaos ).
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