ANNIVERSARY ADDRESS OF THE PRESIDENT. 73 



the earth's history and at distinct points of the surface, renders it 

 impossible to accept the evidence of the larger groups as a whole. 

 Neither the evidence of the freshwater fishes nor that of land and 

 freshwater mollusca as to distribution can be viewed in the same 

 light as that of mammalia, birds, or reptiles. In the latter cases all 

 are homogeneous to that extent, at all events, that we are probably 

 dealing with descendants of one terrestrial form, and there can be 

 little doubt that all fresh groups have diverged from one centre ; in 

 the former case there may have been several centres and several 

 ancestral stocks. In order to analyze the evidence afforded by 

 freshwater fishes and land mollusca, we must take separately each 

 family or other subdivision confined to land or fresh water. 



Land and freshwater mollusca are probably for the most part very 

 ancient, and but for two circumstances would aiford invaluable 

 evidence as to ancient distribution of land-areas. The two difficulties 

 are : — (1) that we are too imperfectly acquainted with the animals of 

 the majority of the species in the most important order of all, the 

 Pulmonata, to classify them correctly ; and (2) that the mystery of 

 the means of migration by which some of them are transported across 

 the seas is unsolved. The prevalent idea that land-mollusca or their 

 eggs are transported by floating logs appears to me extremely 

 improbable in a great number of forms, because, so far as is known, 

 very few either hybernate in wood, or lay their eggs there ; and as 

 the wood is carried to the sea during floods, caused by heavy rains 

 which would certainly make every snail leave its hiding-place, the 

 notion that some would remain ensconced in the clefts appears quite 

 opposed to the habits of the animals. A few shore-haunting forms, 

 such as Truncatellidce or Auriculidce, might very possibly be thus 

 transported, but not Helicidce, as a rule, and still less Cyclophoridm^ ^ 

 the majority of which are very rarely or never seen on trees or 

 wood. 



The smaller forms and their ova are possibly, as Wallace in 

 his later works has suggested, transported by wind, sometimes 

 attached to dried leaves. This may account for the wide distri- 

 bution of a small form like Diplommatina, which lives amongst 

 dead leaves. Eut both the animals and eggs of many forms are ill 

 suited for wind-transport; some, like Acavus, have round or oval 

 calcareous eggs of considerable weight, not easily carried into 



* The genus Lepfo_poma is an exception, as it is said to live on trees. But its 

 distribution is also exceptional, some of its species being found widely dispersed 

 in oceanic islands. 



/2 



