The thesis statement in the passage's introductory paragraph is: "However improbable it may seem, the arrival of around two million Yiddish-speaking Jews from Central and Eastern Europe in North America, between the middle of the nineteenth century and the middle of the twentieth, has turned out to be one of the most profoundly influential modern migrations."
To find the thesis statement, you should look for a sentence that presents the main argument or central idea of the passage. The thesis statement typically appears at the end of the introductory paragraph and provides a roadmap for the rest of the essay or article.
In this case, the sentence "However improbable it may seem, the arrival of around two million Yiddish-speaking Jews from Central and Eastern Europe in North America, between the middle of the nineteenth century and the middle of the twentieth, has turned out to be one of the most profoundly influential modern migrations" stands out as the thesis statement because it presents the central idea of the passage, which is the significant impact of Yiddish-speaking Jews' migration to North America.