Human history is a chronicle of migration. We populated the world by moving out of Africa and reaching every corner of the globe. Throughout recorded history (and surely through all of unrecorded history), human populations have migrated due to war, famine, disease, natural calamity, and in search of better opportunity. One thing that has always traveled with these migrating populations is the opprobrium of their hosts. With a few exceptions, migrants - especially in large numbers - have aroused the natural xenophobia of settled populations. The idea that the diversity introduced by the migrants is, in fact, a force for renewal is both modern and unnatural for the human lizard brain - as described in this wonderful review of four books.