It is difficult to avoid bringing into a paper of this kind the cause of origin of cancerous growths, since cause and prior lesion are so intimately related. Without some sort of stimulus or disturbance of cell relation many of the lesions would never become malignant. Whether a group of cells has been left out of place in fetal life and ready for aberrant growth, or gets misplaced later, can make no great difference for the purposes of this paper, but the latter occurrence seems best to harmonize with the facts. That mechanical, thermic, actinic, chemical, and perhaps local toxic influences often seem the positive exciting cause of malignant development can not be doubted. What the element is in cell relations that holds the balance in so many cases and loses its control in a few, we do not know.
Of all skin lesions tending to cancerous development there are none