Oscar Hertwig (1849 - 1922), a German zoologist and professor, had a significant impact on biology by focusing on the commonalities of all living things, which had predominantly explored the diversity and differences in nature up to that point. He published numerous textbooks and monographs in which he expressed his perspective. In his two-volume work "Current and Controversial Issues in Biology," Hertwig impressively addresses the questions regarding the development of organisms that were controversially debated at the time. This is a reprint of the original edition from 1897.