272 THE ORIGIN OF SPECIES 



themselves from tnese small enemies would be able to 

 range into new pastures and thus gain a great advan- 

 tage. 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 to existing species 

 in nearly the same state, although now of very slight 

 use; but any actually injurious deviations in their struct- 

 ure would of course have been checked by natural selec- 

 tion. 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 

 prehension, or as an aid in turning, as in the case 

 of the dog, though the aid in this latter respect must 

 be slight, for the hare, with hardly any tail, can double 

 still more quickly. 



In the second place, we may easily err in attributing 

 importance to characters, and in believing that they have 

 been developed through natural selection. We must by 

 no means overlook the effects of the definite action of 

 changed conditions of life — of so-called spontaneous varia- 



