What arguments can make against nativism and anti-immigrants feelings?

It is through cultural mixing and immigration that cultures move forward, Alexander the Great unified a great empire through Cultural Diffusion.  The United States itself was created with immigrants and pilgrims.

Yes you are cute answer theories to explain the colonization of the new world the last great habitable landmass to be occupied by humans focus on the bering land bridge, or beringia, which emerge by siberia and alaska the last ice age.