4 o8 ELEMENTARY ZOOLOGY 



and the sensitive probing-bill of the snipe are adaptations 

 connected with the special feeding habits of these birds. 

 The quills of the porcupine, the poison-fangs of the rattle- 

 snake, the sting of the yellow-jacket, and the antlers of 

 the deer are adaptations for self-defence. The fins and 

 gills of fishes, the shovel-like fore feet of the mole, the 

 wings of birds and insects and bats, the toe-pads of the 

 tree-toad, the leaping-legs of the grasshopper, all these 

 are adaptations concerned with the special life-surround- 

 ings of these animals. 



Adaptations may relate to habits and behavior as well 

 as to structure. Plainly adaptive are such habits as the 

 migration of birds and some other animals, most of the 

 habits connected with food-getting, and especially striking 

 and interesting those connected with the production and 

 care of the young, including nest-making and home- 

 building. 



Species-forming. It is evident that through the cumu- 

 lative action of natural selection, animals of a structural 

 type considerably (even unlimitedly) different from any 

 original type may in time be produced by the gradual 

 modification of the original type under new conditions. 

 If, for example, a few individuals of a mainland species 

 should come to be thrown as waifs of wave and storm 

 upon an island, and if these should be able to maintain 

 themselves there and produce young, increasing so as to 

 occupy the new territory, there would be produced in time 

 a new type of individual conforming or adapted to the 

 conditions obtaining in the island, these conditions being, 

 of course, almost certainly different from those of the 

 mainland. Thus as an offshoot or derivation from the 

 original type still existing on the mainland we should 

 have the new island-inhabiting type. Now when these 

 island individuals come to differ so much, structurally and 

 physiologically, from the mainland type that they cannot, 



