We discuss N. I. Vavilov’s 1922 landmark publication, “The Law of Homologous Series in Variation,” and highlight its salient points. Vavilov drew upon his considerable experience in the field with wild and cultivated crop plants and summarized what he noticed in the form of a law. The law stated that comparable variant forms tended to appear in different varieties of the same species, different species of the same genus, different genera of the same family, and so on. His survey of (...) the older literature suggested that the law applied to multicellular animals as well. Vavilov acknowledged that such “parallel” series might be the outcome of convergent evolution (as conventionally understood). At the same time, by way of an alternative hypothesis, he posed a question that continues to engage evolutionary biology to this day. Namely, does the existence of such parallelisms reflect underlying structural and developmental principles (based on the laws of physics and chemistry) which constrain the range of possible plant and animal forms? Vavilov’s original 1922 paper from the _Journal of Genetics_ (12:47–89) is available as supplementary material in the online version of this article. (shrink)