246 ON VARIABILITY AND ADAPTATION 



cell substance, and either themselves act as enzymes, or more 

 probably form compounds of a new type within the cell which 

 act as side chains of a new order, and are capable of building up 

 new side-chain molecules in series, the excess of which become 

 discharged as antitoxin molecules. 



However, from the pursuit of such ideas I again must desist, 

 for I draw you into the depths. I only wish to impress upon you 

 that the known existence and extent of intracellular enzyme 

 activity and the development of the same afford us a full physio- 

 logical basis for the assumption by the cell of habits of 

 activity. 



Thus far I have discussed only those habits associated appar- 

 ently with no anatomical or histological change in the parts 

 that are involved in the habit. This is, however, by no means 

 necessary, and the last few minutes of this discourse may be 

 spent in calling attention to the habit of growth conditions 

 associated with distinct structural changes in the affected tissues. 

 The whole somewhat limited group of what we term metaplasias 

 are really growth habits. They are conditions in which, always, 

 it would seem, under the influence of altered environment, 

 one or other tissue undergoes a metamorphosis, and continues 

 to develop in the metamorphosed state. I had a beautiful 

 example of this only a few months ago, in which, apparently 

 under the influence of chronic inflammation, it could be seen 

 that the perichondrium of the tracheal cartilages had given 

 origin to or had undergone conversion into cartilage cells within 

 its substance, which cells, undergoing proliferation, had now 

 projected outward into the submucosa to form small nodular 

 growths, and these in their turn had undergone bony transforma- 

 tion. Here, then, one and the same tissue, under different 

 conditions of environment, gave rise to connective tissue, cartil- 

 age, and true bone. Nor was this all. These multiple bony 

 nodules projected into the lumen of the trachea, and, doing 

 this, interfered with both the nutrition and the function of the 

 overlying epithelium. As a result, over each nodule the mucosa, 

 instead of being formed of a single palisade layer of columnar 

 cells, had become converted into a well-formed squamous epi- 

 thelium, while between the nodules was the normal columnar- 

 celled epithelium. 



The cells here under different environment had assumed a 



