The differentiation of science into its specialties is, after all, an artificial and man-made state of affairs. While the level of knowledge was still low, the division was useful and seemed natural. It was possible for a man to study astronomy or biology without reference to chemistry or physics, or for that matter to study either chemistry or physics in isolation. With time and accumulated information, however, the borders of the specialties approached, met, and finally overlapped. The- techniques of one science became meaningful and illuminating in another.In the latter half of the nineteenth century, physical techniques made it possible to determine the chemical constitution and physical structure of stars, and the science of "astrophysics" was born. The study of the vibrations set up in the body of the earth by quakes gave rise to the study of "geophysics." 'Me study of chemical reactions through physical techniques initiated and constantly broadened the field of "physical chemistry," and the latter in turn penetrated the study of biology to produce what we now call "molecular biology.