256 Biology in America 



selection may step in to eliminate a species which is not 

 adapted to one or the other of them. But as we have just 

 seen there is no correlation between differences in the snails 

 themselves and amount of difference in their surroundings. 

 Nor do the possible enemies of the snails differ in their dis- 

 tribution in the different regions. 



Isolation however cannot of itself have caused these dif- 

 ferences which must have arisen by spontaneous variation of 

 the snails themselves, due to factors as yet unknown ; isola- 

 tion playing merely a preservative role in maintaining the 

 variations tluis originated, which tend to increase by "ortho- 

 genesis," for reasons equally obscure. 



For several years past Dr. Paul Baartsch of the Smith- 

 sonian Institution has been carrying on an interesting and 

 significant experiment, when taken in conjunction with the 

 studies of Gulick and Crampton on the snails of the Pacific 

 Islands. Baartsch has employed a genus (Cerion) of land 

 snails found in Florida, the Bahamas, Porto Rico and neigh- 

 boring islands. Several colonies of these have been trans- 

 planted to the Florida Keys, and while the plantings have 

 not in every case been successful, many have thrived and 

 after some years in their new home, the snails in some in- 

 stances have shown marked differences in size from the orig- 

 inal type. His results are as yet too incomplete to permit of 

 generalization however. 



Out of all the haze of evolutionary theory do any facts loom 

 large against the background of the past? We may I think 

 safely say that evolution itself is such a fact ; that organisms 

 tend to vary for reasons in the main as yet obscure, but un- 

 doubtedly due in the last analysis to the influence of environ- 

 ment, either internal or external to the organism itself ; while 

 natural selection and isolation play an important, but sec- 

 ondary role, by preserving those variations which are fitted 

 to survive. 



