Every fauna, every… the animal life of a given area is always a mixture of things that have immigrated from various sites. Now, I did an analysis of the birds of North America for instance, and showed which ones had come in at an early time from Asia, at a later time from Asia. I didn't point out… because that's the one thing… the plate tectonics has changed our thinking… I didn't point out that there was a colonization across the North Atlantic, but I also analyzed how much of a colonization there had been from South America and vice versa, how much of a colonization of South America had taken place from North America. And then later on I followed this up once more and did it by families and established for each family where it had come from, whether from the tropical or the temperate zone part of North America, or had come all the way from Asia, and I think the basic analysis that I did in these two papers has never been questioned or changed.