176 ON THE ORIGIN OF SPECIES 



America absolutely depends on their power of resisting 

 the attacks of insects : so that individuals which could 

 by any means defend themselves from these small 

 enemies, would be able to range into new pastures and 

 thus gain a great advantage. It is not that the larger 

 quadrupeds are actually destroyed (except in some rare 

 cases) by flies, but they are incessantly harassed and 

 their strength reduced, so that they are more subject 

 to disease, or not so well enabled in a coming dearth 

 to search for food, or to escape from beasts of prey. 



Organs now of trifling importance have probably in 

 some cases been of high importance to an early pro- 

 genitor, and, after having been slowly perfected at a 

 former period, have been transmitted in nearly the 

 same state, although now become of very slight use ; 

 and any actually injurious deviations in their structure 

 will always have been checked by natural selection. 

 Seeing how important an organ of locomotion the tail 

 is in most aquatic animals, its general presence and 

 use for many purposes in so many land animals, which 

 in their lungs or modified swimbladders betray their 

 aquatic origin, may perhaps be thus accounted for. 

 A well-developed tail having been formed in an aquatic 

 animal, it might subsequently come to be worked in for 

 all sorts of purposes, as a fly-flapper, an organ of pre- 

 hension, or as an aid in turning, as with the dog, 

 though the aid must be slight, for the hare, with 

 hardly any tail, can double quickly enough. 



In the second place, we may sometimes attribute 

 importance to characters which are really of very little 

 importance, and which have originated from quite 

 secondary causes, independently of natural selection. 

 We should remember that climate, food, etc., probably 

 have some little direct influence on the organisation ; 

 that characters reappear from the law of reversion ; 

 that correlation of growth will have had a most im- 

 portant influence in modifying various structures ; and 

 finally, that sexual selection will often have largely 

 modified the external characters of animals having a 

 will, to give one male an advantage in fighting with 



