306 



Le Conte's original work. The mandibles are visible or not, according as 

 they are exposed or withdrawn, and their existence may depend on the 

 sex, as, so far as my material justifies conclusion, they are visible in the 

 male only. Where found, they correspond to Le Conte's description. 

 Even in the larva they are weak and of doubtful service in mastication, 

 while in the imago they are, as is also the labrum, quite rudimentary; 

 which fact hardly justifies us, however, in arguing their non-existence. 



As confirmatory of the affinities of Platypsyllus, as here proved^ it 

 may be mentioned that Leptinus testaceus Mtill., the only species of its 

 genus, is known to be parasitic on mice, as it has been found upon them 

 in Philadelphia by Dr. John A. Eyder, and I have taken it in the nests 

 of a common field mouse near Washington ; but still more interesting 

 is the fact that Leptinillus validiis Horn (also the only species of its 

 genus) is an associate parasite of Platypsyllus on the beaver, a number 

 of both having been taken by one of my agents, Mr. A. Koebele, in 

 San Francisco, from beaver skins brought from Alaska. 



In reference to the classificatory value that 

 should be attached to an aberrant type like this 

 I have already expressed my opinion in a paper 

 on Megathymus, a Lepidopteron that connects 

 in many ways the two great divisions of butter- 

 flies and moths, published in the Transactions 

 of the Academy of Sciences of Saint Louis, Vol- 

 ume III, 1876, and will take the liberty of read- 

 ing a few passages therefrom : 



Between all classificatory divisions, from variety to king- 

 dom, the separating lines we draw get more and more 

 broken in proportion as our knowledge of forms, past and 

 present, increases. Every step in advance toward a true 

 conception of the relations of animals brings the different 

 groups closer together, until at last we perceive an almost 

 continuous chain. Even the older naturalists had an ap- 

 preciation of this fact. Linnseus's noted dictum, " Nat lira 

 saltus nonfaGit" implied it; and Kirby and Spence justly 

 observe that " it appears to be the opinion of most modern 

 physiologists that the series of atitinities in nature is a 

 concatenation or continuous series ; and that though an 

 hiatus is here and there observable, this has been caused 

 either by the annihilation of some original group or spe- 

 cies, or that the objects required to fill it up are still in 

 existence but have not yet been discovered." 



Modern naturalists find in this more or less gradual 

 blending their strongest arguments in favor of commu- 

 nity of descent; and speculation as to the origin, or out- 

 come rather, in the near present or remote past, of existing forms is naturally and very 

 generally indulged, even by those who a few years back were more inclined to ridicule 

 than accept Darwinian doctrine. Shall we then say that the old divisions must be dis- 

 carded because not absolute? A.s well might we argue for the abolition of the four 

 seasons because they differ with the latitude, or because they gradually blend iuto 

 each other. Eutomologists will always speak of moths and butterflies, howsoever 





Fig. 70.— FuLl-yrown larva 

 from below— greatly enlarged 

 (after Eiley). 



