246 OEGAXS OF LITTLE BIPOETAXCE [Chap. VI. 



(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 

 progenitor, and, after having been slowly perfected 

 at a former period, haA*e 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 

 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 swim- 

 bladders 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 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, 



