408 THE MAINTENANCE OF SPECIES 



of the air in some form of nitrate which would combine with the life 

 elements found in sea water and the carbon dioxide of the air ? This 

 theory is in reality a refurbished concept of spontaneous generation. 

 In discussing it two points should be kept in mind. First, if sponta- 

 neous generation of this sort did occur at one time, the contrast between 

 the physical environments of the past and present would be great. 

 Second, even if conditions were right for the similar production of 

 life today, it appears likely that such simple beginnings would be 

 almost immediately destroyed by better established forms of life. 

 Both serve as explanations of why we do not have life produced 

 spontaneously today. 



Life Produces Life 



Since the time of William Harvey, court physician of Charles I of 

 England, the statement "Omne vivum ex ovo" has been used. Living 

 things come from other living things, not always from eggs, as Harvey 

 said, but in the case of unicellular animals and plants by the cell 

 dividing to form two. 



Each organism, plant or animal, has a definite life cycle, a series of 

 changes which it goes through from its simplest form as an egg to its 

 ultimate adult structure. More than this, sooner or later it will die. 

 In some unicellular forms the life cycle takes a very brief period 

 indeed, but in the elephant it is over a hundred years, and some 

 trees, like the giant sequoias, live thousands of years. Sooner or later 

 life activities cease and the Biblical statement of "dust to dust" is 

 justified. Death comes as a final close of all activity and normally 

 after the animal or plant has produced offspring. 



New individuals, whether complicated mammals or simple protozo- 

 ans, arise from the same kind of pre-existing organisms. The exact 

 method of reproduction, however, varies markedly in different 

 groups. Protozoa, at one end of the scale, produce new individuals 

 by the simple process of cell division, while the mammals, at the other 

 extreme, show evidence of considerable division of labor with special 

 organs involved in the production and functioning of the highly 

 specialized sex cells. In order to understand these various processes 

 it is desirable to summarize the different reproductive devices which 

 appear in the animal kingdom. 



Regeneration 



The replacement by an organism of lost or injured tissue is included 

 in this discussion of reproduction on the ground that such a phe- 



