ANNIVEESARY ADDRESS OP THE PRESIDENT. 7 I 



It is reasonable to suppose that all mammalia and all birds have 

 spread from one part of the world in each case, whilst the carps 

 may have originated in one continent at one geological epoch, and- 

 another family of freshwater fishes, for instance the GharacinidcB or 

 Ohromididce, at another place at a different epoch. This fact 

 manifestly has an important bearing on distribution ; for the 

 original dispersal and evolution of each group must have depended 

 on the position and connexion of land-areas at the time. 



At the same time, when the whole of a family, as in the case of 

 the carps or of the Ci/dostomatidce, is exclusively freshwater or 

 terrestrial, it is highly probable that all members of that family are 

 descended from one original marine type, and this probability is 

 frequently borne out by the geographical distribution of the family. 

 Thus carps (Cyprinidse) abound throughout Huxley's Arctogsea, 

 the Palsearctic, Ethiopian, Oriental, and l^earctic regions of Sclater 

 and Wallace, but are absent in Australia and South America. 



As in the class Pisces, so in the subkingdom Mollusca, the fresh- 

 water forms belong to widely different groups. Thus the Unionidce 

 and Paludinidoe, both purely freshwater families, belong to two 

 widely different classes, the Lamellibranchiata or Pelecypoda and the 

 Gasteropoda. Here too, as amongst the fishes, we find some families 

 entirely confined to fresh water, as the examples mentioned above ; 

 others marine with freshwater genera, as the liissoidm or Hydrohiidce 

 with Bythinia, Littorinidoi with Cremnioconchus, Mytilidce with 

 Dreissensia and Byssanodonta, and finally genera like Neritina^ 

 with some species marine or estuarine, others fiuviatile and even 

 inhabiting mountain -torrents. 



Amongst land-mollusca, although there is by no means the same 

 diversity as in the freshwater members of the subkingdom, there are 

 nevertheless several families of very different afiinities. The 

 families are, as a rule, entirely terrestrial, but they are frequently 

 allied to other families that are marine. The most important forms 

 belong to the order Pulmonata, including the Helieidce, Limacidce, 

 Testacellidw, and several other purely terrestrial families, the 

 LhnneldcB and Physidce freshwater, the Aurieulidce, Oncidiidce, and 

 AinpJiiboUdce essentially brackish water or estuarine, but with 

 marine representatives, and in the Auriculidce with at least one truly 

 terrestrial genus Camjjtonyx, and lastly the marine Siplionariidce. 

 The estuarine and marine types are, however, without exception 

 littoral, and the whole order may be as thoroughly terrestrial in 

 origin as mammals. Very different is the case with the land-shells 



VOL. XLVI. f 



