CHAPTER 34 



The Concept of Evolution 



The preceding chapters have served as an introduction to the immense 

 variety of forms of life which inhabit every conceivable place on land 

 and in the water, and exhibit tremendous variations in size, shape, de- 

 gree of complexity, and methods of obtaining food, of evading predators 

 and of reproducing their kind. How all these species came into exist- 

 ence, how they came to have the particular adaptations which make 

 them peculiarly fitted for survival in a particular environment, and why 

 there are orderly degrees of resemblance between forms which permit 

 their classification in genera, orders, classes and phyla, are fundamental 

 problems of zoology. From the detailed comparison of the structures of 

 living and fossil forms, from the sequence of the appearance and extinc- 

 tion of species in times past, from the physiologic and biochemical 

 similarities and differences between species, and from the analyses of 

 heredity and variation in many different animals and plants has come 

 one of the great unifying concepts of biology, that of evolution. Evolu- 

 tion is not a new topic at this point, for it has been fundamental, both 

 implicitly and explicitly, to many of the subjects discussed previously. 



295. The Principle of Organic Evolution 



The term evolution means an unfolding, or unrolling, a gradual, 

 orderly change from one state to the next. The planets and stars, the 

 topography of the earth, and the chemical compounds of the universe 

 have undergone gradual, orderly changes sometimes called inorganic 

 evolution. The principle of organic evolution, now universally accepted 

 by biologists, simply applies this concept to living things: all the various 

 plants and animals living today have descended from simpler organisms 

 by gradual modifications which have accumulated in successive genera- 

 tions. 



Evolution is continuing to occur; indeed, it is occurring more 

 rapidly today than in many of the past ages. In the last few hundred 

 thousand years, hundreds of species of animals and plants have become 

 extinct and other hundreds have arisen. The process is usually too 

 gradual to be observed, but there are some remarkable examples of 

 evolutionary changes which have taken place within historic times. For 

 example, some rabbits were released early in the fifteenth century on a 

 small island near Madeira called Porto Santo. There were no other 



695 



