HOTANICAL JnOlAXJY. 419 



(luce it. Perhaps, on the analogy of the higher plants, in whicii some 

 of them also occur, we may attribute to the latter category certain bodies 

 closely resembling vegetable alkaloids; these are called ptomaines, and 

 are extremely poisonous. Besides such bodies, Bacteria undoubtedly 

 generate true ferments and peculiar coloring-matters. But there are in 

 most cases of putrefiiction a profusion of other substances, which repre- 

 sent the various stages of the breaking up of the complex proteid mole- 

 cule, and are often themselves the outcome of subsidiary fermenta- 

 tions. 



These results are of great interest from a scientitic point of view. But 

 their importance at the present moment in the study of certain kinds of 

 disease can hardly be exaggerated. I have alread^^ mentioned Henle 

 as having first found the true clue to animal histology in the structure 

 of plants. As early as 1840, the same observer indicated the grounds for 

 regarding contagious diseases as due to living organisms. I will state 

 his argument in the words of De Bary, whose " Lectures on Bacteria," 

 the last work which we owe to his gifted hand, I can confidently recom- 

 mend to you as aluminous but critical discussion of a vast mass of dif- 

 ficult and conrticting literature. 



It was of course clear that contagion must be due to the communica- 

 tion of infectious particles or coutagia. These contagia, although at the 

 time no one had seen them, Henle pointed out, "have the power, pos- 

 sessed, as far as we know, by living creatures only, of growing under 

 favorable conditions, and of multiplying at the expense of some other 

 substance than their own, and therefore of assimilating that substance." 

 Henle enforced his view by comparison with the theory of fermentation, 

 which had then been promulgated by Schwann. But for many years his 

 views found no favor. Botanists however as in so many other cases, 

 struck on the right j^ath, and from about the year 1850 steady i)rogress, 

 in which De Bary himself took a leading part, was made in showing that 

 most of the diseases of plants are due to parasitic infection. The reason 

 of this success was obvious; the structure of plants makes them more 

 accessible to research, and the invading parasites are larger than ani- 

 mal coutagia. On the animal side all real progress dates from about 1860, 

 when Pasteur, having established Schwann's theory of fermentation on 

 an imi)regnable basis, took up Henle's theory of living contagia. 



The only risk now is that we may get on too fast. To put the true 

 theory of any one contagious disease on as firm a basis as that of alco- 

 holic fermentation is no easy matter to accomplish. But I believe that 

 this is (notwithstanding a fiood of facile speculation and imperfect re- 

 search) slowly being done. 



There are two tracts in the body which are obviously accessible to 

 such minute organisms as Bacteria, and favorable for their develof)- 

 ment. These are the alimentary canal and the blood. In the case of 

 the former there is evidence that every one of us possesses quite a little 

 llora of varid forms and species. They seem for the most i)art, in 

 health, to be comparatively innocuous; indeed it is believed that they 



