EVOLUTION OF ANIMALS AND PLANTS 563 



relationship through descent. Thus we classify together the 

 fishes, amphibians, reptiles, birds, and mammals because the 

 general plan of structure is so similar in all these vertebrates 

 as to suggest that they have descended from some common 

 ancestors or primitive vertebrates which lived in the far- 

 distant ages with which geology deals. 



493. Evidences of Evolution. In this book we can con- 

 sider only briefly the lines of evidence which led Darwin and 

 his followers to accept the idea of evolution as a true state- 

 ment of the facts observed in study of animals and plants. 

 These evidences will be outlined under the following para- 

 graphs : Structural Evidences ; Embryological Evidences ; 

 Geological Evidences; Distribution Evidences; and Ex- 

 perimental Evidences. 



494. Structural Evidences of Evolution. In almost every 

 lesson of this book we have had occasion to refer to similarity 

 of structures in organisms. Such similarity is the most re- 

 markable fact in biology ; and the only scientific explanation 

 is that organisms have been evolved or have developed from 

 common ancestors. 



Especially striking are the facts in the case of many ad- 

 aptations. Whales have become adapted to aquatic life, 

 but the lost hind-legs are still represented by rudimentary 

 bones far beneath the skin. Snakes appear to have been 

 derived from ancestors with legs, but only a few species 

 (python, boa) show the rudiments of the bones of the hind- 

 legs. Wingless birds (ostrich, Apteryx) have small rudiments 

 of useless wings. Horses still show the rudimentary bones of 

 two lost toes. And in special books one can find records of 

 thousands of such cases of rudimentary structures which point 

 to relationship with forms in which the organs are fully de- 

 veloped and functional. In short, every known adaptation 

 of animals and plants gives evidence of evolution having 

 occurred, for the adapted structures obviously have originated 

 by change and modification of previously existing structures. 



