384 THE NATURALIST IN NICARAGUA. [Ch. XXI. 



acquire means of concealment or stratagem to elude their 

 enemies ; but, on both sides, the improvement would be 

 progressive until the highest form of excellence was 

 reached. Viewed in this light, the wonderful perfection 

 of mimetic forms is a natural consequence of the selec- 

 tion of the individuals that, on the one side, were more 

 and more mimetic, and on the other (that of their 

 enemies) more and more able to penetrate through the 

 assumed disguises. It doubtless happened in some 

 cases that species, having many foes, have entirely thrown 

 off some of them through the disguises they have been 

 brought to assume, but others they still cannot elude. 



Since Mr. Bates first brought forward the theory of 

 mimetic resemblances its importance has been more and 

 more demonstrated, as it has been found how very largely 

 animal life has been influenced in form and colour by the 

 natural selection of the varieties that were preserved from 

 their enemies, or enabled to approach their prey, through 

 the resemblance they bore to something else. So general 

 are these deceptive resemblances throughout nature, that 

 it is often difficult to determine whether sexual preferences 

 or the preservation of mimetic forms has been most 

 potent in moulding the form and coloration of species, 

 and in some the two forces are seen to be opposed in 

 their operation. Thus in some butterflies that mimic 

 the HeliconidaB, the females only are mimetic, the males 

 retaining the normal form and coloration of the group to 

 which they belong. In such cases it appears as if the 

 females have not been checked in gradually assuming 

 the disguise they wear, and it is important that they 

 should be protected, as they are more exposed to de- 

 struction while seeking for places to deposit their eggs ; 



