246 ORGANS OF LITTLE IMPORTANCE [Chap. VL 



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 

 structure would of course have been checked by natural 

 selection. Seeing how important an organ of locomo- 

 tion the tail is in most aquatic animals, its general pres- 

 ence 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 b.§ 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 spon- 

 taneous variations, which seem to depend in a quite 

 subordinate degree on the nature of the conditions, — of 

 the tendency to reversion to long-lost characters, — of 

 the complex laws of growth, such as of correlation. 



