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In page British Shorthair:

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By the 1900s with the advent of the newly imported Persian and other long-haired breeds, the British Shorthair had fallen out of favour, and breeding stock had become critically rare by World War I.[4] At least partially to alleviate this, British Shorthair breeders mixed Persians, Russian Blue, Burmese, and Chartreux into their bloodlines.[4] Part of the genes thus introduced would eventually become the basis for the British Longhair. At the time, any long-haired cats produced were placed into the Persian breeding program.[citation needed] As all cats with the blue colouration were then judged together as variants on a de facto single breed, the Blue Shorthair, outcrossings of the British with the Russian Blue were also common.[2]