What does the movement of plants and animals from Asia to America indicate? What does the author imply by using the words "boxe

There is no doubt that human came only recently to the Western Hemisphere. None of the thousands of sites of aboriginal habitation uncovered in North and South America has antiquity comparable to that of Old World sites. Human’s occupation of the New World might date several tens of thousands of years, but no one argues he has been here even 100,000 years.
    Deduction as to how man found the way to America was lively at the outset and the proposed routes boxed the compass. However, with one or two notable exceptions, students of American anthropology soon settled for the reasonable idea that the first immigrants came from a land bridge which had connected the northeast comer of Asia to the northwest comer of North America across the Bering Strait. Mariners supplied the reassuring information that the strait is not only narrow but also shallow. With little eels in the way of evidence to sustain the Bering Strait land bridge, anthropologists predicted that man walked dry-shod from Asia to America.
    However, at the end of the last century, it became obvious that the Western Hemisphere was the New World not only for human but also for some animals and plants. Zoologists and botanists showed subjects of their respective kingdoms must have originated in Asia and then spread to America. These findings were neither astonishing nor totally unexpected. Such spread of populations is not to be conceived as an departure or mass migration. It is a spilling into new territory that accompanies increase in numbers. But the enormous traffic in plant and animal’s forms casted a heavy burden on the Bering Strait land bridge as the anthropologists ahead predicted it. Whereas purposeful men can make their way across a narrow bridge, the slow diffusion of animals and plants will need an avenue as a continent and available for ages at a stretch.