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This was endorsed by the Russian Empire , and introduced the convention that would eventually become commonly accepted , but not without criticism by many modern analytical geographers like Halford Mackinder who saw little validity in the Ural Mountains as a boundary between continents .
 Russian Empire, Halford Mackinder, Ural Mountains
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Russian Empire, Halford Mackinder, Ural Mountains