AUTHOR'S PREFACE. 



THE present work is in the main a short abridgement of a 

 number of lectures, some of which were delivered in a connected 

 series as a University course, others as occasional and separate 

 addresses. The form of the lectures has been occasionally 

 altered to meet the difference between a written treatise and free 

 oral delivery accompanied by demonstrations. Some things 

 have been omitted and others added, especially some matters of 

 general importance which were not published or did not become 

 known to me till after the delivery of the actual course. 



The lectures were an attempt to introduce an audience com- 

 posed of persons of very different professional pursuits, medical 

 and non-medical, to an acquaintance with the present state of 

 knowledge and opinion concerning the much discussed questions 

 connected with Bacteria. They had, therefore, to give such a 

 survey of the subject as would be intelligible to all who were 

 not strangers to the elements of a scientific training, and 

 especially to set forth the known facts in the life of the 

 Bacteria in their connection with those with which we are 

 acquainted in other branches of natural history. 



A survey of the present extensive literature of the subject, 

 and of the almost daily additions to it, shows the existence of 

 many serviceable and some excellent publications, but at the 

 same time also of much that is mistaken and obscure. The 

 scientific and semi-scientific converse of the day, if I may use 



