INTEEDEPENDENCE OF ANIMALS. 29 



doubt, evident that the animals now mhabiting Australia are 

 so widely separated from those of England that, irrespective 

 of other circumstances, it would he quite impossible for them 

 to have any influence on each other. But if we turn our 

 attention to a defined region, such as North or South America, 

 where the most widely difierent animals live in the most 

 intimate and ever varying contiguity to each other and to the 

 plants which occur there, the case is altogether different. If 

 the American prairies were to cease to produce grass, the first 

 result would be the rapid and utter extinction of the now 

 numerous hetds of buffaloe ■, and on their existence depends that 

 of the surviving remnant of the ancient Indian population of 

 America. If the various insectivorous birds of North America 

 were exterminated, within a very few years beyond a doubt all the 

 produce of the rich agricultural districts of that continent would 

 be destroyed. If we change the mode of life of any single 

 animal, the change will instantly have an influence on all the 

 other animals whose healthy existence was in any way depen- 

 dent on its normal function before it was altered. Although 

 it is certainly true that the various animals inhabiting a country 

 are not so intimately interdependent as the organs of the 

 individual, the relations in the two cases may be very directly 

 compared. The normal numerical proportion, mode of life, and 

 distribution of animals would be altered or destroyed by the 

 extermination of one single animal, just as the whole body 

 sufiers, with all its organs, if only one of them is destroyed or 

 injured. And in both cases nature has analogous remedies at 

 her command. In the one case the function of the incapacitated 

 .organ can be assumed, at any rate to a certain extent, by some 

 other uninjured organ, exactly as, in the other case, the function 

 of the exterminated animal may be fulfilled with regard to the 

 whole fauna of the country by some other animal. But a 

 perfect compensation for the loss sustained is impossible in 

 either case. 



This paTallel between an individual organism and the con- 

 ditions of distribution at present existing may be carried out 

 with reference to the purely morphological relations. 



Every animal body is constructed to a certain extent in 



