478 THE EVOLUTION OF LIFE. 



inevitably come in contact with circumstances partially 

 different from their previous circumstances; and such of 

 them as adopt the habits of other organisms, necessarily 

 experience re-actions more or less contrasted with the re- 

 actions before experienced. Now if changes of organic 

 structure are caused, directly or indirectly, by changes in 

 the incidence of forces; there must result unlikenesses of 

 structure between the divisions of a race which colonizes 

 new habitats. Hence, in the absence of obstacles to migra- 

 tion, we may anticipate manifest kinships between the ani- 

 mals and plants of one area, and those of areas adjoining 

 it. This inference corresponds with an induction before 

 set down ( 106). In addition to illustrations of it already 

 quoted from Mr. Darwin, his pages furnish others. One 

 is that species which inhabit islands are allied to species 

 which inhabit neighbouring main lands; and another is 

 that the faunas of clustered islands show marked similari- 

 ties. " Thus the several islands of the Galapagos Archi- 

 pelago are tenanted," says Mr. Darwin, " in a quite 

 marvellous manner, by very closely related species; so that 

 the inhabitants of each separate island, though mostly dis- 

 tinct, are related in an incomparably closer degree to each 

 other than to the inhabitants of any other part of the world." 

 Mr. Wallace has traced " variation as specially influenced by 

 locality " among the Papilionidce inhabiting the East Indian 

 Archipelago : showing how " the species and varieties of 

 Celebes possess a striking character in the form of the an- 

 terior wings, different from that of the allied species and 

 varieties of all the surrounding islands ; " and how " tailed 

 species in India and the western islands lose their tails as 

 they spread eastward through the archipelago." During 

 his travels on the Upper Amazons, Mr. Bates found that 

 u the greater part of the species of Ithomice changed from 

 one locality to another, not further removed than 100 to 200 

 miles ; " that " many of these local species have the appear- 

 ance of being geographical varieties ; " and that in some 



