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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