What barriers prevented plants, animals, and people from reaching north america?

  following the agreement that was reached in the treaty of indian springs in 1825, chief william mcintosh lost his life. why did this happen

Which environmental change allowed people to travel by land between asia and north america a. human apaption to arctic environments b. the introduction to horses to north america c. the formation of ice sheets on land d. a rise in ocean temperatures

Which statement is correct? the speed of the light ray is the same in air and medium a. the speed of the light ray is the same in air and medium b. medium a and medium b have different densities because light refracts at different angles. medium a and medium b have the same density because the light ray strikes both at the same angle.