14 THE SCOPE OF ZOOLOGY 



formed by spring rains. If you refer to books of past centuries you 

 may find pictures of various stages of generation of eels from the mud 

 at the bottom of a river, or there may be descriptions of how you can 

 produce mice by mixing stale cheese, old rags, and other such ingredi- 

 ents. As science advanced, however, it became apparent that living 

 organisms could arise only from pre-existing living organisms through 

 one of the methods of reproduction described above. Spontaneous 

 generation was accepted only because there was insufficient information 

 about the method of reproduction of the forms of life involved. As 

 more facts were revealed, all grounds for belief in spontaneous genera- 

 tion were dispelled. Even today, however, one will occasionally find 

 a person who is not informed on the discoveries of science during the 

 past century and who still holds to a belief in some form of spontaneous 

 generation. 



Plants and Animals 



Zoology, we have already learned, is that branch of biology which 

 deals with animal life. If you were shown a horse and a tree it would 

 be easy enough for you to tell which was a plant and which was an 

 animal, but there are some cases where the distinctions are not as easy, 

 and some criteria are needed for differentiating them. Most plants 

 possess that green material known as chlorophyll. We hear much about 

 chlorophyll today because of its extensive use in personal products for 

 human use. You might get the idea from some of the advertisements 

 that the plants produce chlorophyll for the sole purpose of keeping 

 human beings from smelling like human beings. The plants, however, 

 have a much more practical use for this product — it is necessary for 

 the very important process of food manufacture. Most plants do not 

 get their food from outside sources and, therefore, must depend upon 

 the chlorophyll to manufacture it within their own cells. However, 

 plants known as fungi do not have chlorophyll and, therefore, must 

 get their food from outside sources as do the animals. When we com- 

 pare the structure of the cell wall and other features of the fungi with 

 plants and animals, there is no doubt about their classification as plants. 

 Also, there is good reason to believe that at least most of the fungi 

 have descended from ancestors which bore chlorophyll, but have lost 

 it in the process of their adaptation to a different way of life. There 

 are a few simple forms of life which have some characteristics similar 

 to animals, but which have chlorophyll like the plants. These are 

 difficult to classify, but serve as excellent illustrations of the close 

 similarity of the two in the lower levels of existence. 



