A hat-tip to Judea Pearl, whose e-newsletter alerted me to this interesting post on the EvaluationHelp blog. It shows the original sixteen diagrams in Sewall Wright's classic 1921 paper on correlation and causality.
Philip and Sewall Wright were responsible for seminal contributions to the basic notions of instrumental variables estimation and parametric identification, though there is still some debate over their relative contributions to these important concepts.
Wright, S. (1921). Correlation and causation. Part I Method of path coefficients. Journal of Agricultural Research, 20, 557-585.