BIOLOGY. 259 



tinguishable from the parasitic worm, the life of which is neither 

 more nor less closely bound up with that of the infested or- 

 ganism. If there were a kind of diseased structure, the histo- 

 looical elements of which were capable of maintaining a separate 

 and independent existence out of the body, it seems to me that 

 the shadowy boundary between morbid growth and xenogenesis 

 would be effaced. And I am inclined to think that the progress 

 of discovery has almost brought us to this point already. I have 

 been favored by Mr. Simon \vith an early copy of the last pub- 

 lished of the valuable ' Reports on the Public Health, 1 which, in 

 his capacity of their medical officer, he annually presents to the 

 Lords of the Privy Council. The appendix to this report contains 

 an introductory essay * On the Intimate Pathology of Contagion, 1 

 by Dr. Burdon Sanderson, which is one of the clearest, most com- 

 prehensive, and well-reasoned discussions of a great question, 

 which has come under my notice for a long time. I refer you to 

 it for details and for the authorities for the statements I am about 

 to make. You are familiar with what happens in vaccination. A 

 minute cut is made in the skin, and an infinitesimal quantity of 

 vaccine matter is inserted into the wound. Within a certain time, 

 a vesicle appears in the place of the wound, and the fluid which 

 distends this vesicle is vaccine matter, in quantity a hundred or a 

 thousand fold that which was originally inserted. Now what has 

 taken place in the course of this operation? Has the vaccine 

 matter by its irritative property produced a mere blister, the fluid 

 of which has the same irritative property ? Or does the vaccine 

 matter contain living particles, which have grown and multiplied 

 where they have been planted ? Tire observations of M. Chauveau, 

 extended and confirmed by Dr. Sanderson himself, appear to 

 leave no doubt upon this head. Experiments, similar in principle to 

 those of Helmholtz, on fermentation and putrefaction, have proved 

 that the active element in the vaccine lymph is non-diffusible, and 

 consists of minute particles not exceeding one-twenty-thousandth 

 of an inch in diameter, which are made visible in the lymph by the 

 microscope. Similar experiments have proved that two of the 

 most destructive of epizootic diseases, sheep pox and glanders, 

 are also dependent for their existence and their propagation upon 

 extremely small living solid particles, to which the title of 'micro- 

 zymes' is applied. An animal suffering under either of these 

 terrible diseases is a source of infection and contagion to others, 

 for precisely the same reason as a tub of fermenting beer is capa- 

 ble of propagating its fermentation, by 'infection' or 'conta- 

 gion, 1 to fresh wort. In both cases it is the solid living particles 

 which are efficient; the liquid in which they float, and at the ex- 

 pense of which they live, being altogether passive. Now arises 

 the question, Are these microzymes the results of homogenesis, 

 or of xenogenesis ; are they capable, like the Torulce of yeast, 

 of arising only by the development of pre-existing germs; or 

 may they be, like the constituents of a nutgall, the results 

 of a modification and individualization of the tissues of the body 

 in which they are found, resulting from the operation of cer- 

 tain condition's? Are they parasites in the zoological sense, 



