III. 



THE ELEMENTS OF ORGANIC EVOLUTION. 



All the laws of life, whatever their nature, are valid 

 throughout the organic world. They control the life 

 processes of man, those of the lower animals, and those 

 of "our brother organisms, the plants." They extend 

 to each in its degree. The fact that the laws of hered- 

 ity, for example, extend unchanged in essence from one 

 extreme of organic life to another is most vital to our 

 understanding of the nature of life. For such homology 

 as this, for any fact of homology whatsoever, we have 

 found but one cause, the influence of common descent. 



There are many elements or factors which enter into 

 the processes of organic evolution, and they stand in 

 varied relations to one another. It is not possible to 

 make a classification of them in which there shall not 

 be inequality and overlapping of elements. For the 

 purpose of our present discussion we may group these 

 forces and factors under eight principal heads. 



I. Heredity. — This is the " law of persistence in a se- 

 ries of organisms." Throughout Nature each creature 

 tends to reproduce its own qualities and those of its an- 

 cestors. "• Like begets like." Creatures resemble their 

 ancestors. The germ cell specialized for purposes of 

 reproduction is capable in its development "of repeat- 

 ing the whole with the precision of a work of art." 

 Heredity is the great conservative force of evolution. 



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