CHAPTER XXVI 



THE DOCTRINE OF EVOLUTION AND RELATED IDEAS 



478. In the preceding pages the words evolution and devel- 

 opment have been used frequently, but no effort has been made 

 to define or justify them. In all that has been said it has been 

 assumed that the animals now on the earth have come to their 

 present condition of variety and complexity by natural growth 

 and development, rather than by outright creation as we find 

 them now. This has been assumed, not because there can be 

 any complete demonstration of the view, but because it is more 

 in accordance with the facts as we find them than any other 

 theory which has been offered. 



The student is now in a position to study the question of 

 evolution more broadly, and to appreciate a more full statement 

 of it and some of the discoveries and inferences most closely 

 related to it. 



479. The Meaning of Evolution. There is a good deal of 

 haziness in the thought of people generally as to just what the 

 zoologist means by evolution. Evolution is not in any sense a 

 cause; it is a term for a process, for the way in which present 

 conditions have come about. Briefly, the most important ele- 

 ments in the thought of evolution are gradualness and natural- 

 ness. In more detail, the following features may be said to 

 belong to the idea of evolution : 



1 . All life, so far as we can know, has come from pre-existing 

 life. This year's animals are descended from those of last year; 

 they from those of the year before, and so on back. 



2. All animals are subject to change. Offspring are never 

 just like the parents. If given time enough animals may thus 

 change in any degree. New species may come from old by 

 change. 



3. The animals of the present time are descended from 

 simpler, more generalized ones; and these from still earlier types. 



476 



