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, alga 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 Leptothria buccalis.— Robin 
(1866-1873), after studying the development of 
Leptothria, stated that this microbe first appears in 
the form of a micrococcus; then of a moving bacterium, 
resembling B. termo, B. lineolwm, etc., and finally it 
forms the long immovable rod (bacillus), which consti- 
tutes Leptothria buccalis. This mode of evolution, 
supposed to be usual in the genera Bacillus and 
Leptothria, 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 Leptothria buccalis, The recent progress of 
science no longer permits us to allow this identity. 
We have seen that there are, at any rate, two 
