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 

 *" 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 



