One of the new concepts introduced by Virchow was that of "homology". This term means the opposite of "heterology", an expression suggesting that the components of a malignant tumor never occur within the normal healthy organism. In contrast to this, Virchow had the idea that a malignant tumor is not something new, just an abnormal composition ("Heterotopie"; "Heterochromie", "Heterometrie") of components normally existing within the organism. For that reason he emphasized the similarity --or even identity- of tumor calls with normal cells, but he realized that the former behave parasitically.

Virchow's classification of tumors had a histogenetic base and was not based on their behavior. The reason why he rejected the latter was the unpredictability of biological behavior-in some tumors. He was particularly surprised that myxomas and enchondromas ("the morphologically most benign and most local tumors") as he said, in some cases progressed to metastatic disease. Nevertheless, he defined the following criteria to diagnose a malignant tumor: (i) local progression (destruction, ulceration), (ii) local recurrence, (iii) involvement of lymph nodes, and (iv) metastasis to distant organs. Moreover, he assumed a stepwise development of malignant tumors and that there were different grades of malignancy.

What we nowadays call a malignant tumor was designated by Virchow as "Pseudoplasma Aftergebilde, Gewächse oder" (or) "Neoplasmen". He subdivided the group of neoplasms into "histoid" (sarcomas), "organoid"(carcinomas), "teratoid", and "mixed". With regard to the origin of cancer he stressed three points: (i) local injury ("Causa occasionalis"), (ii) local disposition ("Causa praedisponens"), and (iii) systemic disposition ("Dyskrasie"). He was convinced that the local factors were most important. A local disposition was either acquired (e.g. chronic inflammation and scarring) or innate. On the other hand, he also discussed the possibility that cancer is an infectious disease, caused by a "Krebsbazillus [cancer bacterium]".

V irchow also realized that not all tissues had the same tendency to develop a malignant tumor and that this inclination was dependent on their differentiation: "The higher the differentiation the lower the tendency to develop a malignant tumor". Virchow became aware that cancers originate predominantly from surface tissues, but despite the recognition of its epithelial character, he was convinced that "without doubt the connective tissue is the origin of tumors". Some people assume that later he doubted the accuracy of his hypothesis and that this doubt was the reason why the second half of the third volume of his lectures on tumors (" Geschwulst e ") never appeared. A second point where he was mistaken is his explanation of local progression and metastasis. He believed that a tumor produces substances ("Contagion; Miasma") that penetrate into neighboring tissue inducing the same alterations there. This same process was, in his opinion, responsible for the development of metastases in lymph nodes and distant organs. Therefore, the more humor and the more vessels a tumor contains the higher is the probability of metastatic spread.

Despite some weaknesses in Virchow's view of malignant tumors, its cellular concept was revolutionary and some of his statements like the criteria of malignancy and the importance of local alterations are still true. Moreover, he anticipated the importance of molecular biology one century ago. He wrote that a disease always has a material base, but not always on the anatomical level. In the absence of anatomical alterations, Virchow assumed the etiology of a disease to be on the subcellular or molecular level. Around 1900 Virchow summarized the charges of medicine during the 19th century as follows: "...medicine, and particularly its basic science, pathology, has became a biological science. If we succeed to carry this thought over into the new century it would be the base for harmonious and discerning work". He also stated "much time has gone by before physicians discovered the morphology of cancer, but how far we still have to go and how much is still to do!"