92 THE LOCAL EXTINCTION OF MOLLUSCS. 



slight advance in my direction of the two species of Paludina 

 {vh'ipara). They are now divided from my neighbourhood by an 

 insignificant watershed. 



A history of the " old," but geologicalh- recent, marls does 

 not give much evidence in favour of new arrivals since that 

 period ; almost all our present species are found in these 

 deposits. I cannot, however, trace the species 1 have named as 

 new arrivals in any of the Essex lists of the marl fossils, nor 

 have I found them myself; and as they are conspicuous forms, 

 we may safely conclude that they did not live in the neighbour- 

 hood at the time these marls were deposited. I am credibly 

 informed that Helix aspersa does not occur in the older marls, 

 and among the smaller species there may be some other 

 instances. 



We have a few species now that occur in extraordinary 

 abundance — several of the Helices and Zonites, some species of 

 Pupa, and among the Freshwater forms some species of Planovhis. 

 Bythinia, &c. 



In considering the variations and wanderings of Molluscs, 

 many factors enter into the problem, the values of which are not 

 themselves well understood. Thus we say, and on good evi- 

 dence, that a species will remain unvaried and without appreci- 

 able change of distribution for an enormous length of time. 

 Some, possibly, go further in saying that the gregarious habits 

 of Molluscs tend to this fixity in that it gives facility for per- 

 petual intercrossing, and so keeps them bound to an average 

 type. But, on the other hand, we are acquainted with numerous 

 variations which may also, to a certain extent, be due to these 

 gregarious habits. We cannot tell how these variations are set 

 up — whether it is due to the preservation of some variation in a 

 diminishing colony, or whether it is due to the change of food 

 and environment in a full colony. Again, as to their wander- 

 ings. Like plants, they are sometimes found liaving a time of 

 introduction, culmination, and decay. Some spread themselves 

 over a whole tract of country ; others confine themselves strictly 

 to a circumscribed locality. We conjecture that the supply of 

 food is directly concerned in these wanderings, and the more one 

 notices the habits of Molluscs, the more one becomes impressed 

 with the idea that snails are very fastidious creatures ; it is 

 certain that they will in some cases prefer starvation, or rather 

 hibernation, amounting to death, rather than make any material 

 change in their diet or be greatly disturbed in their habitat. If 



