CHAP. 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 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 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 r 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 Bengger 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 would 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.” 
