PREFACE. vii 
false notions must be replaced by those which are 
sounder and more just. 
For this reason, we have endeavoured to make 
this work intelligible to all. It may be read with 
profit by those who possess the elementary notions 
of natural science which are included in the course 
of. primary instruction. We therefore hope that the 
volume may find a place in the libraries of secondary 
instruction, and in public libraries. 
Although the work is not specially intended for 
physicians, yet practical men may not be indisposed 
to glance at it: it may, at any rate, serve as an intro- 
duction to the much more learned works of Cornil 
and Babés, of Duclaux, Klein, Koch, Sternberg, ete. 
We have given an important place to the botanical 
question, which is too often neglected in works on 
microbian pathology. From this point of view, the 
narrow bond which connects bacteria with ferments 
and moulds has to some extent marked out the plan 
we have adopted; namely, that of passing from the 
known to the unknown, from what is visible with the 
naked eye to that which is only visible with the aid 
of the microscope. 
Angers, September 10, 1885. 
