2 MICEOBES, FEEMENTS, 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 difier much from those which may 

 be produced in true plants, as, for in&tance, in the 

 mimosa. 



Many of the lower plants, belonging to the groups 

 of Algse 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 



