562 APPLIED BIOLOGY 



that all organisms which are now on the earth have been 

 derived from and consequently are directly related to organisms 

 which previously existed in the past history of the earth ; 

 and that similarities or homologies of structure are due to 

 common descent from earlier forms. To give a concrete 

 illustration : the similarities of existing vertebrates are be- 

 lieved to mean that they have descended from an ancient 

 type of animal which had the general plan of body ( 344) 

 now common to all existing vertebrates ; the similarity of 

 existing birds that they are the direct descendants of an an- 

 cient type; and so on for all groups of animals. Hence, all 

 vertebrates are more or less closely related to each other, the 

 degree of relationship being indicated by the closeness of 

 similarity (e.g., a fish and a mammal distantly related ; a dog 

 and a wolf closely). 



492. History of Evolution. This idea that animals have 

 been derived from other animals is a very old one dating back 

 to some philosophers among the ancient Greeks, but it was not 

 developed in a thoroughly scientific form before the nineteenth 

 century. In 1859 Charles Darwin, of England, published 

 a book entitled " The Origin of Species," in which he mar- 

 shalled such a convincing array of facts to prove the evolu- 

 tion of animals and plants that the theory was soon accepted 

 by most scientific men. Following Darwin's suggestions, 

 hundreds of scientific men began to investigate all the facts 

 which seemed to be connected with evolution, and the result 

 is that the theory has been universally accepted in the scien- 

 tific world as the only explanation for the remarkable simi- 

 larity of organisms which has so often attracted our attention 

 in previous chapters. To-day there is no famous living 

 botanist or zoologist who does not believe in the theory of 

 evolution; that is, that species of animals and plants have 

 originated by descent with modification of pre-existing forms. 

 The entire scheme of classification of animals and plants 

 which is now in use by all biologists is based on the idea of 



