238 PASSALID^. 



power is sacriticed, but exact balance is gained. Since it is 

 obvious that the asymmetrical condition is the earlier one, it 

 aj^ijears that the assumj^tion that symmetry camiot be pro- 

 duced from asymmetry is unjustified, and that, when the 

 conditions rendering asymmetry advantageous no longer 

 exist, change in the direction of restoration of symmetry may 

 occur. The two male specimens of Calcodes burmeisteri 

 represented upon Plate I in this volume, illustrate graphically 

 the occurrence of such a progression in the Lucanid^. By 

 bringing together long series of specimens, all vstages in the 

 progression can be shown, the similarity of the smallest males 

 to the females and the dissimilarity of the largest making it 

 quite clear that the latter are in the most recent stage. 



We are as yet quite ignorant of the cause of asymmetry m the 

 Passalid^, but the conditions which have jjroduced this 

 result seem to have been operative only in a certaiu part of the 

 Oriental and Papuan Regions, and there is no apparent reason 

 why their disappearance or relaxation should not have led to 

 the gradual reappearance of symmetrical forms. If, as seems 

 to have been the case in the Languriid^, the as^anmetry 

 accompanied a particular adaptation of the mandibles, the 

 Passalid genus Aceraius, the only one in which the two 

 mandibles show any considerable difference, may be the only 

 one in which those conditions are still fully operative, and 

 others may exhibit different stages of the return to sj'mmetry 

 and thvis be in a later instead of an earlier stage of development. 

 On the other hand, if the resemblance between the Ceylon and 

 Australian genera is not the result of convergence but of 

 actual affinity, other methods of distribution than migration 

 by land may conceivably have been responsible for their wide 

 separation. It is not suggested that Dr. Gravely 's hypothesis 

 must be rejected, Init the construction of insect genealogies is 

 apt to suggest a degree of actuality which, in the nature of 

 things, they camiot possess. The names of known genera and 

 species must be used to represent unknoAvn and extinct forms, 

 and, unless accepted with every reserve, serious misconceptions 

 may be conveyed. 



Structure of the Imago. 



In strong contrast with the highly i)olymoi'i)]iic Lucanid^, 

 the members of this group are characterized by a remarkable 

 uniformity in their outward asjiect. Two only are knoAni in 

 which a red patch relieves the monotony of their colouring. 

 All the rest are jet black, although the lower surface of the 

 abdomen may be red, and many have a clothing of coarse 

 reddish hair. The red s])ecimens often found are those still 

 in the soft immature state in which they emerged from the 

 pupal skin. In the general shape, structure of the head, legs 



