CHAPTER XVII 



HEREDITY AND EVOLUTION 



A fire-mist and a planet, 



A crystal and a cell, 



A jelly-fish and a saurian, 



And caves where the cave-men dwell, 



Then, a sense of law and beauty, 



And a face turned from the clod ; 



Some call it Evolution, 



And others call it God. 



Cakruth 



Although every species of animal tends to produce young 

 resembling itself, no two individuals even of the same species 

 are ever precisely alike. That the young of each species 

 tend to resemble their parents, we say is due to heredity; 

 that they never exactly resemble them, we say is due to 

 variation. 



Variations. As a rule, the differences between animals of 

 the same species are slight. However, an occasional indi- 

 vidual different from any of its kin in certain respects may 

 make its appearance. This difference may be conspicuous, 

 such as a change in color, or very inconspicuous and affect- 

 ing only some small organ or structure of the body. The 

 abrupt appearance of individuals showing differences from 

 their relatives is spoken of as mutation, and the individual 

 is called a mutant or a sport. 



Artificial Selection. Animal breeders have known for a 

 long time that many of the ordinary differences and the 

 mutations which occur in their flocks and herds are passed 

 from one generation to the next. In fact, they have used 

 both these kinds of variations in producing more desirable 



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