It is unnecessary to say that the modern classification of morbid growths owes its foundation and a large part of its superstructure to Virchow, whose classic work, _Die Krankhaften Geschwülste_, showed the direction which future investigators were to pursue and the nature of the discoveries likely to result. The tumor represents the result of the growth of a tissue or tissues which are like or resemble those which form the normal constituents of the body. Although a new formation is present, it is composed of tissues lying within the possibilities of the individual. A new formation of feathers, as Virchow suggests, is beyond the productive powers of human tissues, though within those of feathered animals. A goose can produce a tumor containing feathers, not one in which hairs are found; in the human species tumors containing hairs may occur, not those, however, in which feathers are present. Although the cells of the tumors of man may deviate in their appearances from the cells of normal tissues, this deviation is never so extreme that their analogue cannot be met with in some part of the body. As the normal tissues originate from pre-existing tissues, so the pathological tissues of the tumor grow only from the antecedent tissues. The matrix from which the tumor arises is a normal tissue. There is produced from it, as a neoplasm, either a tissue which follows the type of the maternal tissue, a homologous tumor, or one which deviates in type from that of the matrix, a heterologous growth. Although the latter differs in its composition from that of the matrix, it does not vary essentially from a like tissue to be found elsewhere in the body. It occurs where it does not belong either in place, time, or quantity. The homologous tumor appears rather as a hypertrophy of the tissue from which it arises, and the line between this variety of growth and a simple hypertrophy is often purely arbitrary. Although tumors, in the more limited sense, are solid, fleshy masses, the new formation of tissues may result in the presence of a tumor within which is a cavity with various contents. Such a cavity is not a mere hole, but has a distinct wall of connective tissue lined with epithelium or endothelium. A distinction is thus drawn between cysts and growths--one which is of daily importance in the practice of medicine--and Virchow's oncology includes the consideration of the two varieties of tumors.