ORGANS AFFECTED 207 



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 progenitor, and, 

 after having been slowly perfected at a former period, have 

 been transmitted to existing species in nearly the same date, 

 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 swimbladders be- 

 tray 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 fl.y-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 im- 

 portance 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 condi- 

 tions of life, — of so-called spontaneous 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 cor- 

 relation, compensation, of the pressure of one part on an- 

 other, etc.,— and finally of sexual selection, by which charac- 

 ters of use to one sex are often gained and then transmitted 

 more or less perfectly to the other sex, though of no use 

 to this sex. But structures thus indirectly gained, although 

 at first of no advantage to a species, may subsequently have 

 been taken advantage of by its modified descendants, under 

 new conditions of life and newly acquired habits. 



If green woodpeckers alone had existed, and we did not 



