129 [Vol. XXXV, 



For instance, it very often happens that insular isolation, or 

 isolation on a mountain, has had a much greater effect than 

 transference on a continent. The conclusions to he drawn 

 therefrom may be entirely upset by the discovery of a most 

 aberrant and much more specialized type on a continent, which 

 type may be, moreover, living under conditions apparently 

 little different from those of allied species. Let us take the 

 case of the gigantic land-tortoises, confined in recent times 

 to two isolated island-groups. It might be, and at one time 

 was thought, that these creatures attained their gigantic 

 development on these islands on account of their lack of 

 enemies. The discovery, however, in both recent and ancient 

 strata, of numerous fossil forms of tortoises, some quite as 

 large and in many instances larger than those at present 

 existing, has shown that these great forms were world-wide, 

 and that they have merely survived on these islands, but 

 did not originate there. 



It is a curious fact that on oceanic islands, where certain 

 conditions prevail, there is a strong tendency shown in birds 

 of many widely-separated groups to lose not only the power 

 of flight, but in many cases the development of the sternum 

 carrying the flight-muscles. Again, certain groups also 

 have lost portions of the actual bony structure of the wing, 

 or have the whole wing degenerating in size. We find 

 on the islands south of New Zealand, parrots, ducks, 

 and rails becoming quite powerless as regards flight ; while 

 in New Zealand itself , in recent times, there existed a whole 

 series of birds incapable of flight, in which the actual 

 wings, as well as the shoulder-girdle, had disappeared. On 

 the Mascarene Islands pigeons, parrots, ducks, rails, and 

 several other birds were found to be entirely deprived of 

 the power of flight. On the other hand, in various orders 

 of animals similar cases are found, which apparently owe 

 their parallel development to the necessity of searching for 

 food rather than to factors in their immediate environment. 

 I refer to the penguins among birds, seals among mammals, 

 and turtles among reptiles. In each of these the limbs have 

 been transformed from instruments of flight or ambulation 



