2 MICROBES, FERMENTS, AND MOULDS. 
minute living things have at all times been at a loss 
to decide whether they have had to do with animals 
or plants. There can be no such doubt when we com- 
pare a tree of which the roots are fastened in the soil 
with a quadruped which moves freely on its surface. 
But these are highly developed forms, the one in the 
vegetable, the other in the animal kingdom. The 
lower representatives of the two kingdoms are, on 
the other hand, often so much alike as to baffle the 
most experienced naturalist. The animals which are 
assigned to the order of Zoophyta, or animal-plants, 
have, as the name indicates, a form which led them to 
be for a long while regarded as plants ; many of them 
are fastened to the bottom of the sea or to rocks as if 
by actual roots, and, when superficially examined, their 
movements do not differ much from those which may 
be produced in true plants, as, for instance, in the 
mimosa. 
Many of the lower plants, belonging to the groups 
of Alge and Fungi, live in the water without being 
fixed by roots; many are animated by more or less 
apparent motion, at any rate during part of their 
existence, so that it is often somewhat difficult to dis- 
tinguish them under the microscope from those beings 
which are generally called Infusoria, and which are 
true animals. 
Hence it follows that the boundary between the 
animal and vegetable kingdoms remains indefinite, 
and that many of those microscopic organisms which 
