THE ORIGIN AND HISTORY OF ANIMAL LIFE 



603 



sideration; even if it were true, it does not 

 explain the origin of life, but only how life 

 reached the earth. 



Physicochemical theory 



A number of theories have been proposed 

 to account for the origin of life on the earth, 

 during the period when the earth was cool- 

 ing down from its original incandescent con- 

 dition. The dominant theory now is that 

 when environmental conditions became suit- 

 able for life, certain molecules became or- 

 ganized into the first living system. Even 

 now, life might conceivably arise from non- 

 living matter if the various elements con- 

 tained in protoplasm were to unite in the 

 proper quantities, in the proper relations to 

 one another, and under favorable conditions; 

 but actually we have no real evidence of this. 



However, this concept of the origin of 

 life is made more plausible by recent re- 

 search on viruses. These are the smallest liv- 

 ing things, actually submicroscopic, and 

 much smaller than the smallest bacteria 

 (Fig. 427). Viruses have the characteristics 

 of living organisms for they are able to 

 grow, multiply, and even undergo muta- 

 tions. Some animal diseases such as polio- 

 myelitis (infantile paralysis) are caused 

 by viruses. An interesting fact is that several 

 viruses have been crystallized like inorganic 

 substances. Of special interest is the fact 

 that the tobacco mosaic virus has been sepa- 

 rated into two nonliving substances, a pro- 

 tein and nucleic acid; when reunited, the 

 combined substances had the characteristics 

 of the living virus and caused infection in 

 tobacco plants. However, such viruses do not 

 appear to be the ultimate transitional form 

 between living and nonliving matter, since, 

 as they are known today, they can reproduce 

 only in living cells. 



Nevertheless, many think that the first 

 primordial living particle was some kind of 

 a simple viruslike particle. Of course, virus- 

 like matter, to constitute the bridge between 

 living and nonliving things, must have been 



able to multiply in an inorganic environ- 

 ment, or in an environment of relatively 

 simple organic compounds. 



Where did the drama of the origin of 

 life begin? Because the remains of the 

 earliest forms of animal life are found in 

 rocks which formed in oceans, and since 

 the body fluids of all animals contain cer- 

 tain salts found in sea water, it seems logical 

 to infer that life began in the sea. 



We can only speculate on when life 

 originated. There is fossil evidence of life 

 500 million years ago, and since many groups 

 were well differentiated at that time it is 

 possible that life existed a billion years ago. 



Our final conclusion must be that we now 

 are not sure how, where, or when life be- 

 gan. 



INTERRELATIONS OF THE 

 PHYLA OF ANIMALS 



Invertebrates 



This is a subject about which there has 

 been much controversy. All zoologists seem 

 to agree that the phylum Protozoa belongs 

 at the foot of the "family tree" (Fig. 430); 

 and although the amoeba is the type usually 

 studied first, the flagellates seem to have 

 given rise to the other three classes of proto- 

 zoans. The sponges probably evolved from 

 flagellated colonial protozoans of the Pro- 

 terospongia type (Fig. 428), but they are 

 believed to be an off^shoot from the family 

 tree and not in the main line of evolution. 



Among the important difficulties encoun- 

 tered in attempting to determine relation- 

 ships between phyla are the modifications 

 that have taken place in both adult and 

 larval stages. The result is that what may ap- 

 pear to be an ancestral condition may be 

 nothing more than an adaptation to en- 

 vironment. On this account we can only 

 speculate instead of arriving at definite con- 

 clusions. At any rate, the lower metazoans 

 possess a planula stage in their life cycle, 



