This is a source book unique in its scope, clarity, and general interest. Its 116 excerpts range in time from Epicurus (ca. 300 B.C.) to the turn of the present century and sometimes, when continuity requires, a little beyond (as to K. S. Lashley, 1929). It includes excerpts from Kepler (1604) on the inverted retinal image, Descartes (1650) on the soul's interaction with the machine of the body, Newton (1675) on the seven colors of the spectrum, Locke (1700) on association of ideas, Whytt (1751) on the spinal reflex, Weber (1834) on Weber's law, Darwin (1859) on evolution, Sechenov (1863)...