When Avery, MacLeod, and McCarty identified DNA as the "transforming principle" in the early 1940s, they resolved a fundamental problem in biological science. At the same time their discovery launched the field of molecular biology and marked the start of the contemporary era in genetics. After reading their paper, many scientists changed the focus of their research to further investigate nucleic acids. One of the most successful, Erwin Chargaff, examined DNA using the recently developed techniques of paper chromatography and ultraviolet spectroscopy and found the composition of DNA to be constant within a species but to differ widely between species. This led him to conclude that there must be as many different types of DNA as there are different species.