CHAPTER XXV 

 THE ORIGIN OF ANIMAL SPECIES 



THE origin of the immense number of species of plants 

 and animals inhabiting the earth has been a matter of 

 speculation among naturalists and philosophers for many 

 centuries. One theory has held that each species was 

 created separately, while the other, known as the Theory 

 of Evolution, maintains that living forms are derived by 

 natural processes of descent from species that inhabited 

 the earth in earlier times ; that is, the ancestral forms 

 became extinct owing to changing conditions of climate, 

 food supply, enemies, and other factors, and their de- 

 scendants in the course of many generations have become 

 modified in bodily structure and function, these changes 

 leading to the development, or evolution, of the numer- 

 ous species now living. The evidence in favor of the 

 latter theory is so strong that it is now accepted by 

 scientific men as the true explanation of the mode of 

 origin of all known organisms. 



Although the idea of evolution has been more or less 

 definitely held by various naturalists since the time of 

 Aristotle (384-322 B.C.), others, even as recently as Lin- 

 naeus (1707- 1778) and Cuvier (1768-1832), have insisted 

 that species are immutable, or unchanging in character- 

 istics. Bonnet (1720-1793) was the first among later 

 zoologists to suggest that variations of climate, nourish- 

 ment, and other features of the environment might pro- 

 duce new species, and to use the term evolution in its 

 modern sense ; but he adduced no important facts to 



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