12 COLLEGE ZOOLOGY 



4. The Origin of Life 



No one knows when and where life originated on the earth. 

 Many of the ancients believed that animals were created by 

 divine providence, but this theory of special creation is not 

 accepted by present-day zoologists. Historically the special cre- 

 ation theory was followed by that of spontaneous generation. Ac- 

 cording to this theory animals were supposed to originate directly 

 from inorganic substances; for example, frogs and toads from 

 the muddy bottom of ponds under the influence of the sun, and 

 insects from dew. The brilliant experiments of Redi (1668), 

 Pasteur (1864), and Tyndall (1876) overthrew this theory com- 

 pletely, and scientists now believe that living organisms originate 

 only from preexisting organisms. Where life first began is still 

 unknown, but the meeting point of sea and land is the most 

 probable place of origin. From here the fresh water, deep sea, 

 and land were gradually peopled. 



5. The Cell and the Cell Theory 



(1) Structure. — It has already been noted that the body of an 

 animal is divided up into microscopic units called cells, and that 

 each cell is a small mass of protoplasm containing a nucleus. Cells 

 vary in size and form; some are extremely small, e.g. blood 

 parasites, whereas others, like the egg of a bird, are very large. 

 They have no definite shape, but may be columnar, flat, spher- 

 ical, or long and thin (Fig. 46). The number of cells in a com- 

 plex animal is enormous; there are about 9,200,000,000 in the 

 gray matter of the human brain. On the other hand, certain 

 animals (Protozoa) consist of but a single cell. The size of the 

 animal does not depend upon the size of its cells, but upon their 

 number. 



Figure 2 shows the essential structure of a cell. The largest 

 part of the contents is the cytoplasm. Within this substance is 

 embedded a nucleus. At certain stages in the life activities of 

 the cell an attraction-sphere enclosing one or two centrosomes is 



