1892.] 



THE BRITISH NATURALIST. 



256 



accounted for, in my opinion, by the West Indies being an unfavour- 

 able climate for Clansilia, and the same causes which have prevented 

 the genus extending itself there have kept the single species to its 

 original form." 



Writing on ferns, Mr. E. G. Lowe says : " In the wild state 

 abnormal forms are found most commonly where, from various causes, 

 ferns do not grow luxuriantly, i,e., grow under difficulties. . Where, 

 ferns flourish in a high degree, it is almost useless to hunt for abnormal, 

 forms." The above opinions seem greatly at variance. Probably it 

 would not be far wrong to say that although " difficulties,' that is, 

 unusual surroundings, must certainly tend to promote variation, still 

 the surroundings may be so peculiar as to be tolerated by one form 

 only, which, of course, may or may not be the type- 

 Mr. G. W. Shrubsole records a curious modification of P/. coriieiis, 

 several of his examples having '*the aperture widely trumpet- 

 shaped," and this in his opinion, "has arisen on the part 'of'"t'he' 

 animal from a desire to cover over as much as possible of the ravages 

 caused by erosion," and he adds " In this instance we are able to 

 assign the varietal character to a physical cause." 



A more striking case of variation is recorded by Mr. W. Doherty 

 in an early number of the Conchological Society's "Journal." Writing 

 from Cincinnati, he says : " The eastern part of the Union is the 

 peculiar habitat of gastrodont or internally dentate species of Zonites, 

 and in a gastrodont variety of Zonites (Conolus) fnlvus^ Drap., recently 

 found at several points near Cincinnati, we have an example of a 

 widely distributed species, spread over all the northern parts of 

 Asia, Africa, and America, assimilating in one portion of its range to 

 the forms prevalent there." The advantages of these dentate 

 processes, Mr. Doherty believes to consist in obstructing the entrance 

 of the shell against the intrusion of a grub " which lives in beds of 

 leaves and preys on small snails — by entering the shell at its mouth 

 and devouring the animal. " The denticles may have been evolved 

 as a protection against foes of this description." 



From the foregoing facts we may, I think, conclude not only that 

 almost infinite variations occur in species, some of which become 

 fixed, and eventually form new species, or survive, whilst the type 

 becomes extinct, but that organisms possess possibilities of devolop- 

 ment which certain conditions of environment are able to bring into 

 action as required. 



In these notes I have endeavoured to bring together, with some 

 attempt at arrangement, the various facts related and opinions 

 expressed regarding the causes of variation by a largc^ number of 



