president's addeess. 79 



palseontological history of any hint of passage forms between the 

 classes themselves, or between the Mollusca and their nearest allies. 

 Nor is this hiatus confined to the Molluscan phylum ; it is the same 

 for all branches of the animal kingdom. There is circumstantial 

 evidence that transitional forms mast have existed, but of actual proof 

 none whatever. All the classes of Mollusca appear fully fledged, as it 

 were. No form has as yet been discovered of which it could be said 

 that it in any way approached the hypothecated ptorhipidogiossute 

 mollusc, still less one linking all the classes.^ 



Howbeit, behind the period yielding the earliest fossils known to us 

 must have lain an age equally vast as that separating us from it, and 

 we can only hope that in some yet geologically unexplored region 

 of the earth's surface, examples of older rocks may be found that have 

 escaped the metamorphic agencies which have rendered ours azoic. 

 At present we can only dimly infer from such available material as we 

 have what the connecting links between these forms of life may have 

 been in the past. 



Perhaps, by way of conclusion, a word or two as to the possible 

 ancestry of the Molluscan phylum may not be out of place. 



The subject has had a good deal of fascination for the morphologists, 

 but dealing with recent material and mostl)^ being unfamiliar with the 

 palaeontological aspect and its requirements, the conclusions they have 

 arrived at have been rather various. 



The two principal theories are (1) that the Mollusca were derived 

 from the Flat-AVorms of the Class Turbellaria, and (2) that they come 

 from the Segmented Worms (Annulata), while recently R. T. Gunther 

 {19) has sought to show that the Chsetognatha are the prhnitive 

 Mollusca. This last hypothesis has been met by Thiele {50) with 

 arguments which though brief seem effective, quite apart from the 

 fact that it does not seem practicable to compare so highly specialized 

 a being as a Chsetognath with such equally highly differentiated 

 Molluscan forms as the Dibranchiate Cephalopods and the Pteropoda. 



The Turbellarian ancestry of the phylum has been advocated by Lang 

 {27), Thiele (^.9, pp. 5i)7-508), and others, Thiele more particularly 

 indicating the Polycladia Cotylea {Ij.9, p. 529) as the stem. Korschelt 

 and Heider admit that the theory has much to be said for it, but 

 consider " this origin has the disadvantage of starting from very 

 highly differentiated animals, and . . . affords no explanation 

 of the striking resemblance existing between the larvae of the 

 Mollusca and those of the Annelida" {25, pp. 320-321). Pelseneer, 

 however {36, p. 368), and Garstang {18, pp. 39-44) had already, 

 as it seems to me, sufficiently disposed of the arguments urged in 

 favour of a Polyclad ancestry, the latter postulating a common 

 stem whence the Polyclads branched off on one side and the Annelida 



W. K. Brooks (5a) has developed a somewhat fantastic idea to the effect that the 

 earliest animals were pelagic, that the principal groups evolved them, and that 

 they subsequently discovered and colonized the sea-floor when they became 

 fossilized. This "leaves out of account the fact that pelagic animals drop to the 

 bottom and become fossilized also. 



