CHAPTER IV. 



USE OF THE MICROSCOPE IN ZOOLOGY. 



IN the last chapter I asked the question, "What is 

 the difference between a plant and an animal?" a 

 question more easily asked than satisfactorily answered, 

 for when we examine very low organisms, we seem to 

 touch the confines of the two kingdoms ; but these 

 confines are very difficult to determine indeed, some 

 scientific men have denied any absolute distinction 

 between the two kingdoms. They assert that, not- 

 withstanding the manifold differences in form and 

 structure, there is a " physical basis of life underlying 

 all the diversities of vital existence;" that "a three- 

 fold unity namely, a unity of power or faculty, a 

 unity of form, and a unity of substantial composition 

 does pervade the whole living world." According to 

 that eminent biologist, Professor Huxley, the formal 

 basis of all life is protoplasm, simple or nucleated ; 

 and in the lowest plants, as in the lowest animals, 

 a single mass of such protoplasm may constitute the 

 whole plant, or the protoplasm may exist without 

 a nucleus. How, then, it is asked, is one mass of 

 non-nucleated protoplasm to be distinguished from 

 another? Why call one plant and the other ani- 

 mal? The only reply is that, so far as form is 

 concerned, plants and animals are not separable, and 

 that in many cases it is a mere matter of con- 

 vention whether we call a given organism an animal 

 or a plant. There is a living body called &thalium 

 septicum, which appears upon decaying vegetable sub- 

 stances, and in one of its forms is common upon 

 the surfaces of tan-pits. In this condition, it is to 



