226 ISLAND LIFE 



— Having thus shown that the physical changes of the 

 earth-'s surface may have gone on much more rapidly and 

 occupied much less time than has generally been supposed, 

 we have noAv to inquire whether there are any considera- 

 tions which lead to the conclusion that organic changes 

 may have gone on with corresponding rapidity. 



There is no part of the theory of natural selection which 

 is more clear and satisfactory than that which connects 

 changes of specific forms with changes of external con- 

 ditions or environment. If the external world remains 

 for a moderate period unchanged, the organic world soon 

 reaches a state of equilibrium through the struggle for 

 existence ; each species occupies its place in nature, and 

 there is then no inherent tendency to change. But almost 

 any change whatever in the external world disturbs this 

 equilibrium, and may set in motion a whole series of 

 organic revolutions before it is restored. A change of 

 climate in any direction will be sure to injure some and 

 benefit other species. The one will consequently diminish, 

 the other increase in number ; and the former may even 

 become extinct. But the extinction of a species will 

 certainly affect other species which it either preyed upon, 

 or competed with, or served for food ; while the increase 

 of any one animal may soon lead to the extinction of some 

 other to which it was inimical. These changes will in 

 their turn bring other changes ; and before an equilibrium 

 is again established, the proportions, ranges, and numbers, 

 of the species inhabiting the country may be materially 

 altered. The complex manner in which animals are 

 related to each other is well exhibited by the importance 

 of insects, which in many parts of the world limit the 

 numbers or determine the very existence of some of the 

 higher animals. Mr. Darwin says: — "Perhaps Paraguay 

 offers the most curious instance of this ; for here neither 

 cattle, nor horses, nor dogs have ever run wild, though 

 they swarm southward and northward in a wild state ; and 

 Azara and Rengger have shown that this is caused by the 

 greater number in Paraguay of a certain fly, which lays 

 its eggs in the navels of these animals when first born. 

 The increase of these flies, numerous as they are, must be 



