41 



In speaking of an insect imitating or mimicking an object, such 

 as a leaf, twig, bark, etc., it must not be supposed that the use 

 of such terms imply conscious action on the part of the insect. As 

 will presently be shown more fully, there is in all insects a tendency 

 to vary ; by the laws of inheritance, varieties are reproduced, and 

 natural selection does the rest. 



The offspring of all animals exhibit a general likeness to their 

 parents, but individually they vary to a greater or lesser extent, not 

 only one from the other, but also from the parent type. This is 

 well exemplified in the Lepidoptera. If a series of any species in 

 this order is examined, even though such series is formed of in- 

 dividuals of the same brood, it will be found that no two specimens 

 of the series are exactly identical in every particular of structure, 

 colour, and pattern of marking. In the case of species usually 

 considered constant in colour and markings, the points of difference 

 may be so trivial and minute as to escape detection unless the 

 specimens are carefully and critically compared ; but in a species 

 of a polymorphic character, the divergence from the type and from 

 each other is more pronounced, though there is a decided bias in 

 favour of the parents where the series is composed of individuals 

 of one brood. In any case, if variation from the type is of a 

 nature to confer additional protection on the ,orm so varying in any 

 particular locality, then such form will possess an advantage over 

 the type in that locality, and will probably supersede it, for, as 

 Darwin in Origin of Species tells us, it is varieties of the same species 

 and species of the same genus that come into the sharpest conflict 

 in the great struggle for existence. 



On the other hand, if the type is well protected by virtue of 

 assimilation with its surroundings, and none of the varietal forms, 

 which from time to time occur, improve upon the type in this 

 respect, then the type will continue to prevail ; but should the 

 environment of the species become gradually altered in character, 

 then the forms best fitted to exist under the changed conditions, 

 will be perpetuated, and the former type being at a disadvantage 

 will be gradually eliminated. 



In Boartiiia repandata we have a good illustration of a species 

 which is either variable or constant in coloration according to the 

 locality it inhabits and the nature of its surroundings in that locality. 

 Thus for instance, in the Isle of Lewis, one of the Hebrides or 

 Western Islands of Scotland, the species is represented by a small 

 leaden grey form which in colour and style of ornamentation agrees 



