Chap. VI. AFFECTED BY NATURAL SELECTION. 191 



attacks of insects, mif^lit assuredly he acted on by natural se- 

 lection. Tlie tail of tiie g^iraffc looks like an artificially-con- 

 structed fly-flapper; and it seems at first incredible that tliis 

 could have been adapteil for its present purpose by successive 

 slight niodiiications, each better and better lifted, for so triliino; 

 an object as to drive away flies ; yet avc should pause before 

 being too positive even in this case, for we know that the dis- 

 tribution and existence of cattle and other animals in South 

 America absolutely depend on their power of resisting the at- 

 tacks of insects : so that individuals which could by any means 

 defend themselves from these small enemies, would be able to 

 range into new pastures and thus gain a great advantage. It 

 is not that the larger (juadrupeds are actually destroyed (exceiDt 

 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 pre}'. 



Organs now of trifling importance have probably in some 

 cases been of high imjiortance 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 ver}' slight use ; but any actually injurious 

 deviations in their structure will of course have been checked 

 by natiu'al selection. Seeing how important an organ of loco- 

 motion the tail is in most aquatic animals, its general prcsenco 

 and use for many purposes in so many land animals, which in 

 their lungs or modified swim-bladders betray their aquatic ori- 

 gin, 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 Ily- 

 flapper, an organ of prehension, or as an aid in turning, as with 

 the dog, though the aid in this latter respect nuist be slight, 

 for the hare, with hardly any tail, can double cjuickly eTiough. 



In the second ])lace, ^ve may sometimes wrongly attribute 

 im]X)rtance to characters which have originated from quiti; sec- 

 ondary causes, independently of natural selection. We should 

 remember that climate, food, etc., probably have had some, 

 ])erhaps a considerable, direct influence on the organization ; 

 that characters reajjpear from the law of reversion ; that ct>r- 

 relation is an important element of change; and finallv, that 

 sexual selection has often largely modified the external i-Iiarac- 

 ters of the higher animals, so as to give one male an advantage 

 in fighting witli other males, or in charming the female ; and 



