GEOGRAPHICAL DISTRIBUTION OF THE MOLLUSCA. 153 



to that of the most conservative of their opponents. I should 

 write this book to little purpose, if I should not be able to 

 indicate the identity" of origin of many so-called distinct genera 

 or species, and that in these cases, at least, physical causes have 

 been amply sufficient first to produce and then to maintain 

 diversity ; but the proposition that therefore derivation must 

 account for really important existing structural differences, 

 receives little or no support from actual observation, so far as 

 the mollusca are concerned. 



The older authors, and those who still accept the idea that 

 species are the result of the special intervention of Divine Will, 

 have assigned to the genera and species of animals definite areas 

 of distribution, and which are supposed to be more or less inde- 

 pendent of physical causes. The occurrence of similar forms in 

 distinct areas, they will not admit as conclusive of their common 

 ancestrj^, but prefer to call them " representative species " and 

 to marvel that separately created beings (therefore different 

 species) should so closely resemble one another. I will here 

 say plainly, as a result of many years' study of the geographical 

 distribution of the mollusca, that defined limits to the so-called 

 " Provinces " do not exist in nature except where these provinces 

 have been fixed by physical causes alone ; that whatever or 

 wherever may have been the origin of the various genera or 

 species, physical causes alwaj'^s sufficed to account for their 

 subsequent distribution. I accept the existence of a species 

 where theoreticall}^ it has no business to exist as proof that 

 something is amiss with the theory. Geographical provinces of 

 distribution having necessarily been determined through obser- 

 vation of the species composing them, when more extended 

 observation modifies our knowledge, so should our boundaries 

 vary. To illustrate : it was supposed that most of the shells 

 inhabiting the Atlantic Coast of the United States, north of 

 Cape Cod, were divided by it from those living southwards of 

 that peninsula, and Cape Cod was accordingly made the boundary 

 between two marine provinces ; yet the multiplied researches of 

 modern naturalists have shown and are showing that under like 

 physical conditions neither northern nor southern species hesi- 

 tate to cross the line which the closet-naturalist has forbidden 

 them to cross. Deepsea dredgings also inform us that some 

 Arctic mollusca are quite willing to extend southwards where 

 the surface temperature would forbid their habitation, provided 

 that at a greater depth they can secure climatal influences 

 similar to the shore lines of the northern waters. 



In most cases Provinces of Distribution or Areas of Assem- 

 blage of species are physically defined, in fact, although they 

 have been so frequently somewhat arbitrarily constituted : yet 

 we must carefully recognize the distinction between an assem- 



