WALKER ON ORIGIN AND DISTRIBUTION OP MOLLUSCA. 61 



The present range of the European Margaritina above mentioned is 

 one of the most interesting facts brought to light by the study of the 

 recent distribution of our molluscan fauna. 



It is common both upon the Atlantic and Pacific coasts, but is wholly 

 wanting in the Mississippi valley and the interior region lying to the 

 north. That it must have been an early immigrant from the old world is 

 shown the fact, that to have allowed its present range it must have at 

 one time extended clear across the northern portion of the continent. 

 Its total disappearance in the interior region is in all probability to be 

 attributed in common with so many of our faunal peculiarities to the 

 effect of the glacial period. Upon the approach of the ice, there was 

 undoubtedly a retreat, as far as possible, before it of all forms of animal 

 life toward the south. By what means the eastern Margaritinm were 

 enabled to escape from the total destruction which overwhelmed their 

 brethren in the interior cannot be told. But there must have been some 

 factors connected either with the advance of the glacier or the means of 

 retreat, which saved the eastern contingent from extermination and by 

 which upon the subsequent recedence of the ice cap, the survivors were 

 able to regain their former foothold in the northern Atlantic states, al- 

 though prevented by physical changes from spreading again toward the 

 west into the interior region. 



The relations existing between the peculiar, so called "North Ameri- 

 can" types of Unionidce and both the Tertiary fauna of southeastern 

 Europe and the fossil and living fauna of Asia are very interesting and 

 when fully known seem likely to give important data as to the origin 

 of this widely extended family and the manner in which it has attained 

 its present world-wide distribution. But the existing material is as yet 

 too scanty to be used, even for speculative purposes. 



Of the many interesting details of the local distribution of existing 

 species, both in this and other families represented in our fauna, the 

 already too greatly extended limits of this paper forbid mention. 



From the roughly drawn outlines which have been given of the prin- 

 cipal theories now advanced to account for the distribution of our exist- 

 ing fauna and of the main facts upon which they are based, some idea can 

 perhaps be obtained from the nature of the work, which modern science 

 seeks to accomplish, of the measure of success that has already been 

 attained and of the possibilities, which lie before the student, »f the 

 geological and geographical distribution of the mollusca. The enormous 

 advance that has been made within the few years that have elapsed since 

 Darwin and Wallace opened the doors to untrammeled thought and 

 investigation, it is but a foretaste of that, which surely is to come. And 

 while perhaps it is too much to expect that, from the scattered debris 

 of the wreckage of past ages, the chain of animal life can ever be re- 

 constructed in its entirety, there is every reason to believe that, in the 

 years to come, much that is now inexplicable will be made plain and 

 that in its broad outlines, at least, and to a high degree of certainty the 

 true history of its origin and development will be elucidated. 



