president's addeess. 197 



FordiUa, Nucula, Modioloides, Modiolopsis, Pterinea, Posido7iomya, Area, 

 Isoarca, Conocardium, Megalodon, Cardiomorpha, and others offer so 

 mucli variety of form as to lead one to conclude that we are still very 

 far from having arrived at a primordial bivalve. 



Amongst the Scaphopoda, the genus Dentalium occurs fossil in the 

 Lower Silurian, and has survived to the present day. About 160 

 fossil forms and nearly 100 living species, very widely distributed, are 

 known. 



Lastly, of the order Polyplacophora, Chitons have been recorded 

 fossil as far back in time as the Lower Silurian, and are abundantly 

 represented in the seas of to-day. 



I have avoided speaking of the Cephalopoda, as I felt that to deal 

 with that group alone would more than suffice for an entire address. 

 They have, moreover, quite lately formed the subject of an Address by 

 Prof. Blake to the Geologists' Association. I will merely state that in 

 the opinion of the late Dr. J. Barrande the Cephalopods of the earlier 

 rocks offer no sign of evolution, but only of most remarkable persistence 

 in those types which, like Nautilus, have survived ; whilst Goniatites 

 and Ammotiifes appear suddenly, and retain their distinctive cha- 

 racters until their final extinction. Probably later on Mr. Crick will 

 give us his views on the Cephalopoda in relation to their evolution in 

 geological time. 



Since the publication of Darwin's "Origin of Species," in 1859, 

 far more attention has been paid by biologists to questions relating 

 to variation and evolution and the development of living things than 

 had seemed needful to the older naturalists, our immediate pre- 

 decessors, who had inherited a firm belief in the immiitability of 

 species. Thus, the late Edward Forbes writes (" Nat. Hist, of British 

 Seas," p. 8) : "Every true species presents in its individuals certain 

 features, specific charaeters, which distinguish it from everj^ other species; 

 as if the Creator had set an exclusive mark or seal on each type" 

 Indeed, it seems to me that the earlier naturalists looked specially 

 for resemblances, whilst later observers have paid more attention 

 to differences, often even to extremely minute variations. Linnaeus 

 expresses his opinion respecting genera and species thus: '■'■Classis et 

 ordo est snpientim, species naturce opus,'" which may be fi'eely translated 

 ^'■Species are real; genera tdeal.^' "The marshalling of species in 

 classes and orders is matter of discretion ; but species themselves are 

 the work of God." (S. P. Woodward.) 



Pirmly believing as I do in Evolution, I cannot but feel that we 

 seem still to need much more extensive and intimate knowledge of 

 the races of living beings which preceded the present Molluscan fauna, 

 in order to demonstrate clearly their origin and descent. We know 

 that the present races closely resemble their immediate predecessors, 

 and differ more and more from the shells of older geological times. 

 We must, of course, admit the validity of the plea of the imperfection 

 of the Geological Record, yet at the same time (allowing for that imper- 

 fection) we cannot fail to discern a steady onward progress in most 

 organisations, marked, however, by some curious exceptions : — 



1. One fact is apparent from our Geological Record of the past, 



