Die Seele des Kindes. Beobachtungen über die Geistige Entwicklung des Menschen in den Ersten Lebensjahren

The child's soul: Observations on human mental development in the first years of life. 

William Thierry Preyer (1841-1897) was a founder of scientific child psychology, and a pioneer in regards to research of human development based on empirical observation and experimentation. He was inspired by Charles Darwin's theory of evolution and Gustav Fechner’s work in psychophysics.