CHAPTER 36 



The Origin and 

 History of Animal 



Life 



X HROUGHOUT OUT studics of animals, we 

 have considered primarily the species that 

 Hve on the earth at the present time; now 

 we shall discuss questions such as: what is 

 the origin of animal life on the earth, and 

 what is the history of animal life from the 

 earliest known fossils to the present time? 

 This involves: 



1. A review of the hypotheses to account for 

 the origin of life. 



2. A study of the relationships of different 

 phyla to one another, and the relationships 

 of groups within phyla. 



3. An attempt to determine what types of 

 variation occur within a species, and how 

 these variations originate. 



4. A study of the effects of natural selection 

 in relation to these variations, and of the 

 adaptations that result from the struggle 

 for existence. 



5. An examination of the various types of evi- 

 dence indicating that organic evolution, the 

 development of animal life to its present- 

 day forms, has occurred. 



6. A discussion of the principal theories that 

 have been proposed to account for organic 

 evolution. 



7. A brief account of the development of man. 



ORIGIN OF LIFE 



Spontaneous generation 



Previous to the end of the seventeenth 

 century, the theory of spontaneous genera- 

 tion of animals was accepted by both scien- 

 tists and philosophers. According to this 

 idea, many species of animals were supposed 

 to arise spontaneously from nonliving mat- 

 ter. For example, insects were believed to 

 originate from dew, frogs and toads from 

 the muddy bottoms of ponds under the in- 

 fluence of the sun, butterflies from cheese, 

 and fly maggots from flesh. Even today, 

 many uneducated people believe that mos- 

 quitoes are generated by stagnant water and 

 that horse hairs, when placed in water, 

 change into living "snakes." The classical ex- 



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