CTTAP. X.] THE RATE OF ORGANIC CHANGE. 



219 



specific forms with changes of external conditions or environ- 

 ment. 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 atfect 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 materi- 

 ally 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 w^orld limit the numbers or deter- 

 mine 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 north- 

 ward in a wild state ; and Azara and Kengger 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 habitually checked by some means, probably by other para- 

 sitic insects. Hence, if certain insectivorous birds were to 

 decrease in Paraguay, the parasitic insects wovild probably 

 increase ; and this would lessen the number of navel-frequenting 

 flies — then cattle and horses would run wild ; and this would 

 certainly alter (as indeed I have observed in parts of South 

 America) the vegetation : this again would largely affect the in- 

 sects, and this, as we have seen in Staffordshire, the insectivorous 

 birds, and so onwards in ever increasing circles of complexity." 



