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. At least partially to alleviate this, British Shorthair breeders mixed Persians into their bloodlines. 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. 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.