The chromosome theory of inheritance is a cornerstone of modern genetics. It postulates that chromosomes are the carriers of Mendelian factors (genes) and are the physical basis of heredity. This theory, first proposed in 1903, led to the inseparable union of two previously distinct fields of investigation— cytological observation of chromosomes and genetic analysis of inheritance from breeding experiments. It was vital to subsequent rapid advances in genetics and the eventual understanding of the physical and chemical nature of genes. Our ability to think interchangeably about genes and chromosomes is so integral to genetics today that it is hard to conceive that this theory did not gain immediate widespread acceptance. Nonetheless, prior to Bridges’s 1916 investigation, the chromosome theory remained an unresolved issue.