In 1996 Raven, Axelrod, and Al-Shehbaz wrote a paper on the history of the modern flora of China, Europe, and the continental United States. They said that the three regions have approximately the same geographic area, yet China has two times the number of species as the United States, and three times as many as Europe. They asserted that all three regions had essentially the same flora as of 15 million years ago, but China came to possess the most species because of three reasons. First, China has a tropical rain forest. Second, there is an unbroken gradient of vegetation from the tropical rain forest to "boreal coniferous forests that has persisted and afforded habitats characterized by equable climates during the last 15 million years, when massive extinctions were taking place elsewhere in the Northern Hemisphere...such continuity is interrupted in North America by the Gulf of Mexico and in Europe by the Alps, the Mediterranean, and the Sahara Desert." The third reason was due to the impact of the Indian subcontinent with Asia starting 50 million years ago, making a "highly dissected, elevated geography."