CHAPTER 



CHANGING ANIMALS 



The Fact of Change 



We have mentioned (p. 1 ) that organic evolution deals 

 with changes undergone by living things, plants and animals. Some readers 

 who are not used to thinking of these matters may feel that we are making 

 an unwarranted assumption when we speak of animals changing. The fact 

 that they do change has by no means always been recognized. Indeed, 

 until quite recently in the history of human thought most people have be- 

 lieved that the animals living today were created as they now are, once 

 and for all, as recorded in the first chapters of Genesis. This belief was 

 championed by many eminent scientists of former times. Among these was 

 Linnaeus, the eighteenth-century Swedish naturalist who founded the sys- 

 tem of classification still used (see Chap. 14). Linnaeus assigned scientific 

 names to great numbers of plant and animal species and genera. He be- 

 lieved that these species were for the most part the ones created as de- 

 scribed in Genesis. As his knowledge expanded, however, he modified 

 this view to the extent of conceding that new species might arise through 

 hybridization (cross-mating) between the original species. In view of this 

 widespread belief in the fixity of species, how is it that we now speak of 

 animals as changing? In other words, what makes us think that the kinds 

 of animals living today are not the kinds of animals which have "always" 

 existed? 



The direct evidence on the question just raised comes from the "record 

 of the rocks" — from the remains of animals that formerly hved but are 

 now known to us only as fossils. In fairness to Linnaeus we should recall 

 that almost nothing was known about fossils in his day. As we shall see in 

 later chapters (Chaps. 7-1 1), this geologic record demonstrates that hosts 

 of animals not present in the modern world formerly lived. What became 



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