1915.] 195 



Jlbfjtracts of Ilerciit literature. 



BY HUGH SCOTT, M.A., F.L.S., F.E.S. 



Gravely, F. H., "The Evolution and Distribution of certain Indo- 

 AusTBALiAN Passalid Coleoptera." Jouni. and Proo. Asiat. See. Benga], 

 Vol. 10, No. 6, pp. 201-210, pi. 24, 1914. 



In two sub-families of these beetles, Acerniinae and Gnaphalocneminae 

 many species are more or less asymmetrical. in the forni of the front of the head 

 and dentition of the mandibles. The asymmetrical condition appears to have 

 been evolved separately in the two sub-families, and in different divisions of 

 the second, for five distinct types of asymmetry occur. These five types are 

 characteristic of five groups of genera, which are quite distinctly separable 

 from one another by other characters besides those liable to asymmetry. It is 

 strongly apparent that in each group, whether of genera or species, the most 

 highly asymmetrical forms are the dominant forms ; e.g., in a group of species, 

 the most highly asymmetrical siDecies stands out by reason of its abvmdance, its 

 occurrence over the whole area inhabited by the group, its variability in size 

 (this usually exceeding that of the other species), audits gregarious habits: 

 [it is usual in Passalidae for the larvae and both the parents to live together in 

 a family; " gregariousness " here implies the living together of a number of 

 such families]. Moreover, the different genera or groups of genera are with few 

 exceptions confined to well-marked divisions of the region they inhabit ; and a 

 study of their distribution leads to the conclusion that the more highly 

 specialised forms — i.e., in this case, the dominant forms — have migrated outwards 

 radially from the distributional centres, driving before them the less highly 

 specialised forms, which for the most part only maintain themselves in particular 

 areas on the outskirts of the range of the whole group. The boundary-line 

 between the Oriental and Australian regions separates the groups into two sets 

 filmost completely : and in both these regions there seems to have been an 

 evolutional and distributional centre, with subsequent radiating outwards of 

 more highly specialised forms, driving the more primitive ones before them. 



Thei'e is a curious similarity between these phenomena and those exhibited 

 by two other groups of animals in the same region, namely, the Thelyphonidae 

 (curious Arachnids of the group Pedipalpi), and a marine group, the feather- 

 stars or Crinoids. As regards the latter, the great difference lies in the fact that 

 whereas in the Passalids the dominant species of a group is also the most highly 

 specialised, in the Crinoids the dominant species falls in each group about mid- 

 way between the most specialised and most primitive. The difference may, 

 perhaps, be explained by the supposition that in the Passalidae the highly 

 specialised forms are a mature fauna, while in the Crinoids the highly 

 specialised genera are senescent, i.e., no longer dominant, but characterised by 

 possessing but few species, which are very unlike one another, and which occur 

 in widely separated localities. 



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