54 EXPERIMENTS IN THE BREEDING OF CERIONS. 



course, points to a northward movement, and I would not be at all sur- 

 prised, therefore, if fossil ancestors were found in South America, and 

 even fossil members in the chain of the Windward and Leeward Islands. 

 The elements of this chain have at various times been subjected to sub- 

 sidence which may have eliminated the living members. If South 

 America is also the cradle of the Cerions, then the first migration must 

 have taken place at an exceedingly early period — at all events, prior to 

 OUgocene times, for at that period we find them well established in the 

 silex beds at Tampa, Florida. 



The Bahama Cerions offer an interesting field for speculation. 

 Here we find to-day thousands of colonies, sometimes occupying slight 

 elevations separated by slight depressions occupied by shallow water. 

 It appears now that these colonies are perfectly good species. 



Our experiments on Newfound Harbor Key indicate an unusual 

 fertility on the part of the cross between Cerion viaregis and Cerion 

 incanum, and one wonders if such a crossing, where fertile offspring 

 are produced, may not have a rejuvenating effect upon such a hybrid. 

 As stated before, w^herever I have noted a state of flux among mollusks, 

 whether land, fresh-water, or marine, it was found that with this condi- 

 tion there was always associated a very large production of individuals. 

 This is true to such an extent that the fluxed element has always been 

 the dominant form of the group to which it belongs in the region 

 occupied by it. This would suggest that crossing has an energizing 

 effect which seems to enable the new product to surpass its associated 

 congeneric forms in the production of offspring. 



It seems possible that during the glacial period, when the low state of 

 water in the sea united the numerous keys which constitute the Bahama 

 Archipelago into one land mass, forms separated by sea barriers during 

 Pliocene times were enabled to come together and to cross. It is not 

 at all improbable that this crossing may have resulted in an efflores- 

 cence of new forms which may have swarmed over all the shores where 

 we now find the Cerion glans group and probably other groups repre- 

 sented. The close of the glacial period, returning the water to the sea, 

 again split up the region into numerous islands and promontories where 

 the ancestors of the Cerions of to-day were stranded in colonies and 

 where inbreeding gradually eliminated diverse characters, eventually 

 resulting in the more or less homogeneous expression which we now 

 find in each colony. 



