AFFECTED BT NATURAL SELECTION. 187 



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 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 subse- 

 quently come to be worked in for all sorts of purposes, as a 

 fly-flapper, an organ of prehension, or as an aid in turn- 

 ing, 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- 

 tions, 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, comprehension, 

 of the pressure of one part on another, etc., and finally of 

 sexual selection, by which characters of use to one sex are 

 often gained and then transmitted more or less , perfectly to 

 the other sex, though of no use to the 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 



