Until the middle of the 20th century, moving species around the world was reckoned a normal, often valuable, thing to do, while the consequences of their accidental movement were rarely considered.

到20世纪中叶以前，人们一直认为物种在世界各地的迁徙是一件正常的、并且通常极具价值的事情，而很少考虑它们偶然迁徙造成的后果。