CHAP. VI.] AFFECTED BY NATURAL SELECTION. 147 



great, though of a very different kind, as in the case of the most 

 perfect and complex organs. 



In the first place, we are much too ignorant in regard to the 

 whole economy of any one organic being, to say what slight 

 modifications would be of importance or not. In a former chapter 

 I have given instances of very trifling characters, such as the down 

 on fruit and the colour of its flesh, the colour of the skin and hair 

 of quadrupeds, which, from being correlated with constitutional 

 differences or from determining the attacks of insects, might 

 assuredly be acted on by natural selection. The tail of the giraffe 

 looks like an artificially constructed fly-flapper ; and it seems at 

 first incredible that this could have been adapted for its present 

 purpose by successive slight modifications, each better and better 

 fitted, for so trifling an object as to drive away flies ; yet we 

 should pause before being too positive even in this case, for we 

 know that the distribution and existence of cattle and other 

 animals in South America absolutely depend on their power of 

 resisting the attacks 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 advan- 

 tage. 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 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 spontaneous variations, which seem to depend in a quite 



6 



