Four possible categories of cause are examined in respect of the almost total neglect of Gregor Mendel's theory of heredity from 1865 to 1900. From this examination, we conclude that these categories do not adequately account for that neglect, and, therefore, that Mendel's paper must have been incomprehensible to his contemporaries and immediate successors primarily on other, presumably conceptual, grounds. We suggest the possibility that in the latter half of the nineteenth century there was no conceptual distinction between the transmission (...) of an hereditary trait from parent to offspring (genetics) and the subsequent development of that trait in the offspring (embryology). (shrink)