IN'PKODIJCTION. 20 



}cmng, as well as the delicate tasks involved in tiic iiidilieation 

 of numerous burrowinji l)eetles like the Lueanid Sinodmdron 

 and the Copkixje (leseril)e<l in a previous volume of tliis series. 

 The most significant fact concerning the majulibles of niah^ 

 LuCANiD.^ is the relation between their size and that of tlu! 

 insect bearing them. The size of the in.seet determines the 

 degree of development of its mandibles. J^arge mauflibles 

 are found in large-bodied species and specimens, and small 

 mandibles in small sj)eeies and specimens. The ancestors of 

 all the LucANiD^, tJiere is reason to believe, were small 

 insects with mandil)les of normal size, diftering little or not 

 at all in the two sexes, and the great development of the 

 organs in the male has occurred as a concomitant of the great 

 increase that has taken place in the size of the insects. The 

 key to the phenomenon must therefore be sought in the causes 

 that, in the course of ages, produce changes in the size of 

 animals of whatever kind. Had J)arwin been aware of the 

 im])ortant size-relation I have mentioned, he woidd not have 

 written " It seems probable that all these characters (he 

 included the mandibles of male Lucaxida;) have been gained 

 through the same means, namely Sexual selection." It 

 appears to me that Natural Selection, and not any sexual 

 selection, is the method by which existing sexual ditterences 

 have been brought about. In the genus XigidiN.<!, in wliich 

 both sexes have mandibles of a kind usually peculiar to males 

 only, we must suj)pose that these organs, owing to the si)ecial 

 conditions of their life, ])resent no hindrance to the females 

 in the task of oviposition ; whereas in other Lucanid^ 

 mantlibles such as are borne by the male would undoubtedly 

 be an encumbrance to the female. The feature, however 

 acquired, has been transmitted to both sexes in the first case ; 

 in the second, any tendency to its transmission to the female, 

 causing a definite hindiance to the ])er])etuation of the species, 

 has been checked by that means, and the result has been a 

 natural selection of races with a weaker tendency to such 

 inheritance by the female. In the horn-bearing genus 

 Sinodendron, where the mandibles of male and female are 

 alike and )K)th sexes share the tunnelling o])erations, we may 

 su])p()se that the enlargement of the male mandibles whicli 

 has taken ])lace in other genera luis, by the operation of 

 Natural Selection, l)een su})pressed through tlie hindrance 

 which wouUl result in the perfornumce of those o])erations. 

 Conversely we may conclude that tiie fact that in other 

 genera the males take no part in providing for the well-being 

 of their offspring and, their mandibles being imused, the 

 restraining influence of Natural Selection is in consecpience 

 not brought to bear upon them, is a part cause of the generally 

 prevailing hj'pertrophy of the organs in male Lucanids. If, 

 in any particular case, a useful employment had been acquired 



