CHAP. X THE RATE OF ORGANIC CHAXCxE 227 



habitually checked by some means, probably by other 

 parasitic 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 insects, and this, as we have 

 seen in Staffordshire, the insectivorous birds, and so on- 

 wards in ever increasing circles of complexity." 



Geographical changes would be still more important, 

 and it is almost impossible to exaggerate the modifications 

 of the organic world that might result from them. A sub- 

 sidence of land separating a large island from a continent 

 would affect the animals and plants in a variety of ways. 

 It would at once modify the climate, and so produce a 

 series of changes from this cause alone ; but more import- 

 ant would be its effect by isolating small groups of in- 

 dividuals of many species and thus altering their relations 

 to the rest of the organic world. Many of these would at 

 once be exterminated, while others, being relieved from 

 competition, might flourish and become modified into new 

 species. Even more striking would be the effects when 

 two continents, or any two land areas which had been long 

 separated, were united by an upheaval of the strait which 

 divided them. Numbers of animals would now be brought 

 into competition for the first time. New enemies and new 

 competitors would appear in every part of the country ; 

 and a struggle would commence which, after many fluc- 

 tuations, would certainly result in the extinction of some 

 species, the modification of others, and a considerable 

 alteration in the proportionate numbers and the geograph- 

 ical distribution of almost all. 



Any other changes which led to the intermingling of 

 species whose ranges were usually separate would produce 

 corresponding results. Thus, increased severity of winter 

 or summer temperature, causing southward migrations and 

 the crowding together of the productions of distinct regions, 

 must inevitably produce a struggle for existence, which 

 would lead to many changes both in the characters and 



Q 2 



