INTRODUCTION 237 



and front of the head sliows " five separate lines of evohition 

 diverging from some symmetrical or almost sjinmetrical 

 ancestor ", an<l each distinguishing a different group of genera. 

 An accomi^anying diagram shows one of these lines of descent, 

 the origin of which is traced back to the Ceylonese Episphenus 

 moorei, as entirely separated from the other lines, which are 

 traced to an Australian origm. Dr. Gravely regards the 

 widely separated and, as he sujjposes, ancestral forms in]ial)iting 

 Ceylon and Australia, as more closely inter-related than the 

 different asjTiimetrical forms inhabiting the regions which 

 separate them. From this he draws the following conclusion :. 

 " In order to explain the geograj)liical separation of the 

 primitive s;yTnmetncal and closely related forms found in the 

 two regions, by the more highly specialized and less closely 

 related allies of each, it must be supposed that conditions on 

 either side of " Wallace's line " {i.e., the dividing line between 

 the Malayan and Papuan Regions— G. J. A.) are for some 

 reason ])eculiarly favourable to the evolution of highly 

 specialized forms ; and that tliese have migrated outwards, 

 driving before them the less highly specialized, which have 

 rarely siu'vived to the present day except where they have 

 been able to estal)lish themselves behind zoogeographical 

 barriers, that the more recently evolved forms have not yet 

 been able to cross." He regards the asymmetrical forms, that 

 is to say, as possessing greater vigour than their symmetrical 

 fore-rumwrs, but as having failed at both extremities of their 

 geographical range to surmount barriers which proved no 

 obstacle to the latter. 



The closely similar character of the as}7nmetiy in every case, 

 consisting in an extension of the same marginal ])rocesses of the 

 head on one side, nearly always the left, sometimes accomi)anied 

 by very slight differences in the mandibles, renders the theory 

 of five independent origins at the least surprising ; and the 

 fact that it is admitted that most of the genera figurmg in the 

 genealogical chart are not really primitive forms, does not 

 strengthen the case. Dr. Gravely's argument rests entirely 

 upon his assumption that synunetrical forms cannot have 

 been derived from asymmetrical ones, and that, although 

 similarity in asymmetrical forms may be ascribed to con- 

 vergence, that of the synunetrical forms cannot be explained 

 in the same way. 



In LrCANiDJE and other beetles asymmetry between the two 

 mandibles is normal, the teeth upon the ojiposed inner edges 

 not being opposite to one another, and so enabling the edges 

 to come together more elasely and to obtain a firmer grip. In 

 many male Lucanidje the enlargement of the mandibles is 

 accompanied by an exaggeration of the teeth which renders 

 the as}^nmetry very conspicuous ; but at the greatest develop- 

 ment of the mandibles, complete sjTnmetry is found — gripping. 



