300 THE ORIGIN OF SPECIES 



ited both areas: and we almost invariably find that 

 ^wlierever many closely allied species inhabit two areas, 

 some identical species are still common to both. Where- 

 ever many closely allied yet distinct species occur, doubt- 

 ful forms and varieties belonging to the same groups 

 likewise occur. It is a rule of high generality that the 

 inhabitants of each area are related to the inhabitants of 

 the nearest source whence immigrants might have been 

 derived. We see this in the striking relation of nearly 

 all the plants and animals of the Galapagos Archipelago, 

 of Juan Fernandez, and of the other American islands, 

 to the plants and animals of the neighboring American 

 mainland; and of those of the Cape de Verde Archi- 

 pelago and of the other African islands to the African 

 mainland. It must be admitted that these facts receive 

 no explanation on the theory of creation. 



The fact, as we have seen, that all past and present 

 organic beings can be arranged within a few great classes, 

 in groups subordinate to groups, and with the extinct 

 groups often falling in between the recent groups, is in- 

 telligible on the theory of natural selection with its con- 

 tingencies of extinction and divergence of character. On 

 these same principles we see how it is, that the mutual 

 affinities of the forms within each class are so complex 

 and circuitous. We see why certain characters are far 

 more serviceable than others for classification: — why 

 adaptive characters, though of paramo unt jULportance 

 to the beings, are of hardly any im port ance _in^ ^^cU§sit 

 fication; why characters derived from rudimenta ry parts, 

 though of no service to the beings, are often of high 

 classi ficatorv value; andwhy embryo logical characters are 

 often the most valuable of all. The real affinities of all 



