I I 2 THE CAUSATION OF DISEASE. 



What can be more interesting than the mode of growth of 

 squamous epithelioma ? The process is strictly analogous to 

 the development of tubular glands, but I can have no doubt 

 that the affected tissues in this disease, be the exact nature 

 of the process what it may, display potentialities which in far- 

 off ancestors had, in very large measure, an actual existence. 

 Indeed, it may be said that most, if not all, diseases are tinged 

 witli an element of reversion. By this I do not mean that the 

 body in disease reverts, as a whole — that each and all of the 

 tissues take on the exact characters of some particular ancestor 

 — but rather that certain local reversions of the tissues take 

 place. It was just now said that all diseases accompanied by 

 distinct structural change are, without doubt, examples of dis- 

 solution. This dissolution being incomplete (for when complete 

 dissolution takes place we are dealing, not with disease, but 

 with death of the part), it necessarily follows that the tissue or 

 tissues which have thus undergone partial dissolution must 

 possess characters which belonged to a past era of evolution. 

 Such local reversion need not be a perfect reversion, as we 

 shall presently see ; but that the change must, in some degree, 

 be a reversion, appears to me an a priori conclusion. 



We must not, however, leave out of account the part played 

 by evolution in the process of disease. Evolution is not un- 

 frequently observed to follow on the structural dissolution, or 

 even to accompany it. Indeed, it is often by no means easy 

 to separate the two processes. Let us instance the process 

 of inflammation, which affords a very good example of local 

 reversion. 



It very frequently happens that, as a result of inflammation, 

 the affected tissue is converted into granulation or embryonic 

 tissue. This, in fact, always happens when the process is 

 neither so severe as to completely destroy the tissue, nor so 

 slight as to permit of complete absorption of inflammatory 

 products. Now, granulation- tissue belongs to an order common 

 in lowly and remote forms of ancestral life. It, therefore, 

 follows that, when a mature tissue is replaced by granulation- 

 tissue, it reverts, in the truest sense of the word. The 

 question we have to decide is, bow far the process whereby the 

 mature and perfectly evolved tissue has been changed into this 



