II.] INCiriENT STRUCTURES. 41 



erally iiiiist favorable variations as to the length of the neck 

 have been accompanied by some unfavorable variation in 

 some other part, neutralizing the .action of the favorable 

 one, the latter, moreover, only taking effect during these 

 periods of drought I How often must not individuals, fa- 

 vored by i\ slightly-increased length of neck, have failed to 

 enjoy the elevated foliage which they had not strength or 

 endurance to attain ; while other individuals, exceptionally 

 robust, could struggle on yet further till they arrived at 

 vegetation within their reach ! 



However, allowing this example to pass, many other in- 

 stances will be found to present great difficulties. 



Let us take the cases of mimicry among lepidoptera and 

 other insects. Of this subject Mr. Wallace has given a most 

 interesting and complete account," showing in how many 

 and strange instances this superficial resemblance by one 

 creature to some other quite distinct creature acts as a safe- 

 guard to the first. One or two instances must here suffice. 

 In South America there is a family of butterflies, termed 

 JIelico7iidw, which arc very conspicuously colored and slow 

 in flight, and yet the individuals abound in prodigious num- 

 bers, and take no precautions to conceal themselv^es, even 

 when at rest, during the night. Mr. Bates (the author of 

 the very interesting work " The Naturalist on the River 

 Amazons," and the discoverer of "Mimicry") found that 

 these conspicuous butterflies had a very strong and disa- 

 greeable odor ; so much so that any one handling them and 

 S(|ucczing them, as a collector must do, has his fingers 

 stained and so infected by the smell, as to require time and 

 much trouble to remove it. 



Tt is suggested that this unpleasant quality is the cause 

 of the abundance of the Hcliconidne ; Mr. Bates and other 

 observers reporting that they have never seen them at- 

 '" Sec " Natural Selection," chap, iii., p. 45. 



