No. 570] 



TAXONOMY AND EVOLUTION. 



381 



Arthropods and Molluscs being hard provide systematists with 

 a sculpture on which it is easy to detect minute differences in 

 pattern. On the other hand we would remind the conchologist 

 that the external parts are by their very positions most liable 

 to exhibit lesions and weathering, and certainly in the case of 

 Mollusca where the dependence of the exoskeleton upon a spe- 

 cific article of diet (viz., lime salts) is very close, to register 

 "fluctuating variation" according to the constitution of the 

 medium or of the food ingested. 



But here again if a more common practise were made in dis- 

 secting by systematists, variations would be found even in closely 

 allied species making the descriptions complete and often 

 even necessitating the erection of new genera. One of the 

 writers was dissecting an ordinary species when he discovered 

 that the epipharynx was so entirely different in form and struc- 

 ture from the usual type for the genus that, had it been an 

 external character it would long ago have been formed into a 



Karel Thon 1 has demonstrated how in Holothyrida a single 

 internal structure is at variance with the other indications of 

 genetic affinity. A great many similar instances will be immedi- 

 ately called to mind by those who practise dissection. 



Again, if systematists are convinced of the taxonomic value 

 of hard parts how comes it that they need to be reminded that 

 there are hard parts in the internal anatomy as well which tiny 

 so frequently and habitually leave unnoticed? The endoskele- 

 ton of Arthropods, gastric mills, pharyngeal ossicles and carti- 

 laginous supports are all systems which might be profitably 

 studied by the entomologist and carcinologist, while the con- 

 chologist generally proceeds as though the radula and jaw were 

 part of the "mush," as he so inelegantly terms the viscera. 



Geographical Distribution 

 The advent of the morphologist into the particular sphere of 

 systcmatics or the metamorphosis of the systematist into a mor- 

 phologist (it matters not how we put this desirable event) will 

 result in the annexation not only of classification, but also of 

 questions of geographical distribution by anatomy and morphol- 

 ogy. How many pretty theories in geographical distribution 

 iZool. lahb., Bd. XXIII, Syst., pp. 720-21. 



