The complete quote is:
The collision and intermingling of these millions of foreign-born people representing so many nations and creeds make New York a permanent exhibit of the phenomenon of one world.
By this statement, E. B. White is comparing the multi-cultural fabric of New York, with all its millions of citizens representing so many nations, to a sort of World's Fair of humanity. He notes elsewhere that many of the residents of New York have arrived there from somewhere else, whether in the United States or abroad, and put down roots. Their gathering together in one city is a kind of "collision" because it stands to reason that not all groups of people are in constant harmony with one another. The use of the word "phenomenon" is White's way of describing the vast variety of human beings present in this world, representatives of each group of which, he is saying, can be found in this one city.
In statistics provided by Mayor Bloomberg's 2013 report, The Newest New Yorkers, it is noted that "New York's foreign-born population has more than doubled to 3 million--a population that would comprise the third-largest city in the U.S., bested by just New York City and Los Angeles." [p.12] The report goes on to note that approximately six out of every ten New Yorkers are either immigrants or the children of immigrants. [ibid] I think, given these statistics, that White's observation is as true today as it was when he wrote it.