Formulated in 1823, the Monroe Doctrine asserted that the Americas were no longer open to European colonization.

One of the greatest breakthroughs for professional women came in 1973 when the field of banking opened up for them.

Eyespots,themostrudimentaryeyes,arefoundinprotozoan原生动物flagellates鞭毛虫,flatworms扁形虫,andsegmentedworms片段蠕虫.

ThefirstimportantexpositionintheUnitedStateswasheldinPhiladelphiain1876.

Formerly , in the United States, many nurses worked as private duty nurses rather than in hospitals.