274 MICROBES, FERMENTS, AND MOULDS. 



siderable time. It is now almost completely abandoned, 

 and recent researches, made with the view of isolating 

 the alga and fungus which were supposed to co-exist 

 in the lichen, tend more and more to show that the 

 lichen is an independent plant, and not merely an 

 association of two plants of distinct families, algae and 

 fungi. 



Errors of the same kind may occur in the study 

 of microbes, which, from their minute size, their 

 unicellular nature, the rapidity of their growth, the 

 variety of their habitat, and the great resemblance 

 of their form, are still more difficult to distinguish 

 than lichens. Of this we will give some examples. 



Polymorphism of Leptothrix bucc'dis. Robin 

 (1866-1873), after studying the development of 

 Leptothrix, stated that this microbe first appears in 

 the form of a micrococcus ; then of a moving bacterium, 

 resembling B. termo, B. lineolum, etc., and finally it 

 forms the long immovable rod (bacillus), which consti- 

 tutes Leptothrix buccalis. This mode of evolution, 

 supposed to be usual in the genera Bacillus and 

 Leptothrix, is probably exact, and, with some reserve 

 as to the specific identity of the different forms 

 observed by Robin, modern micrographists are dis- 

 posed to accept it. But Robin goes further: he 

 regards the anthrax bacillus as specifically identical 

 with Leptothrix buccalis. The recent progress of 

 science no longer permits us to allow this identity. 

 We have seen that there are, at any rate, two 



