314 Prof. T. Thorell on Dr. BerthaiCs 



tinuing, in this way, to build on the old ground, it would 

 seem that arachnologists might gradually draw nearer and 

 nearer to the point aimed at — a fully natural classification. 

 This point is aimed at by all the different zoological depart- 

 ments, and by Zoology as a whole ; nay, such a classification 

 may be said to be the final end of this science, inasmuch as 

 the " system " is, as it were, a compendium of all that is 

 knowni about the natural objects in question ; and a fully 

 natural system presupposes complete knowledge of their 

 natural history in its whole compass. 



Bertkau's opinion is, on the contrary, that the present ar- 

 rangement of the Order of Spiders must be abandoned, as 

 being fundamentally erroneous, and new principles laid down 

 for the classification of these animals. He says that, in 

 contradistinction to former arachnologists, he has in his new 

 system of classification taken into consideration all the modi- 

 fications in the structure of Spiders that are known to him, 

 laying more stress on the differences in the organs of respira- 

 tion than has been in general the case, and making use of 

 characters taken from the form of the web only in case of 

 need (^, p. 354). The principal difference, in this respect, 

 between the classification proposed by Bertkau and that of 

 other more recent arachnologists would, in fact, seem to con- 

 sist in his having, in characterizing both suborders and 

 families, attributed greater importance to differences in the 

 inner anatomical structure than is generally the case, taking 

 into consideration, in the first place, the different structural fea- 

 tures of the organs of respiration, and, in the second place, the 

 organs of generation. In his characterization of the families, 

 the different shape of the tubular trachea (which are some- 

 times ramified either in the form of a tree or in the form of a 

 bundle, and sometimes quite simple and unramified) plays an 

 important part. Now as the Arachnida may be divided into 

 two great groups, according as they breathe with (tubular) 

 tracheae* alone, or with air-sacs either alone or in combination 

 with (tubular) trachese, it might have been expected that 



* If, as is most generally believed, tLe lamellae of the air-sacs are nothing 

 but moditied ordinary or tubular tracheae, then the Arachnida which 

 breathe with these latter organs must be older than, as they no doubt 

 are inferior to, those which breathe with air-sacs ; some authors, how- 

 ever, regard these last-named Arachnids as the more original forms, and 

 as beino- directly descended from the fossil Eurypterids, the gills of these 

 Crustaceans having been directly transformed into the air-sacs of the 

 Arachnida (the Scorpions). How this supposed change came to pass it is 

 not easy to understand ; in the meantime we possess no less than four 

 different hypotheses for explaining it — one proposed by MacLeod, two by 

 Ray Lankester, and one by Kingsley ! 



