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Chapter 3 



Resemblances and Differences 

 Among Living Things 



THE problem of organic evolution is that of finding an 

 answer to the question, How did the living population 

 of the world come to be what it is? The historical 

 evidence indicates different populations in past ages: the 

 plants and animals have not always been of the same kind 

 as those we see today. What connection is there between 

 the ancient inhabitants and the present-day inhabitants? Are 

 our contemporaries, and we ourselves, the descendants of 

 those former inhabitants, or merely successors? How can 

 we tell whether birds or mammals or plants have descended 

 from ancient forms of life and have become modified in the 

 course of the centuries? There are here really two questions: 

 ( I ) What is the evidence that life has been continuous from 

 the ancient forms to the present time? (2) What is the 

 evidence that one species has changed into another species? 



The Need for Classification 



Every normal child begins at some time in his develop- 

 ment to ask about the objects that come anew to his notice, 

 " What is that? " In most cases the child is satisfied with a 

 name: That is a tree, that is an insect, that is granite, that is 

 a frog, that is a rosebush. As the individual mind develops, 

 as experience accumulates, discrimination grows. Uncles and 

 aunts come to be known apart, a rug is differentiated from 

 a carpet. Trees in general become fruit trees and shade trees, 

 trees that lose their leaves in the fall and evergreen trees, 

 pines and spruce. Insects come to be divided into butterflies 



