CHAPTER XIX 



THE ORIGIN OF SPECIES 



Evolution. Everything in nature is subject to change. No 

 sooner is a form produced or a structure completed than it 

 begins to be modified by many agencies. Ultimately it grows 

 old, slowly deteriorates, and finally disappears. Such changes 

 have always been in existence. We know from the fossil ani- 

 mals and plants found in the rocks that there has been a steady 

 succession of different forms in the world, beginning with the 

 strange and incongruous forms of past ages and ending with 

 the species of the present day. The fact that such forms are 

 embedded in the solid rock shows that the very rocks them- 

 selves have changed since animals and plants first inhabited 

 the earth, and also indicates how very profoundly our planet 

 has been modified since time began. When we realize that 

 the one unchanging feature of existence is change, it is easy 

 to appreciate the fact that the plants and animals of our day 

 are quite unlike those that first inhabited the earth. All have 

 changed Avith changing conditions ; indeed, within the memory 

 of living men some of our flowers and fruits have been greatly 

 modified in this way. Our present species, then, appear to be 

 the latest forms in a long line of descent the last links in a 

 chain that might be traced back to the dawn of creation. The 

 process by which our modern plants have come to be what 

 they are, the steps by which they have changed, little by little, 

 are included in the term " evolution." It is usual to assume that 

 evolution has always progressed from the simple to the com- 

 plex, and so it has in most cases ; but evolution may proceed 

 in any direction useful to the organism, and the best we can 

 say of it is that it works through change. 



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