888 NATURALISED FORMS OP LAND AND FRESH-WATER MOLLUSCA 



Whilst the absence of their natural enemies in the shape of Black- 

 birds, Thrushes, and such like may be another cause helping to 

 produce the effect above-mentioned, inasmuch as the absence of 

 hereditary enemies might in time deaden somewhat those instincts 

 in the animal which cause it to be on the alert for danger. In 

 other words — Given no present necessity for a thick shell as a 

 protection against enemies or cold, together with constant changed 

 environment and altered circumstances (food, climate, times, and 

 seasons, &c.), differing altogether from the ancestral experiences (I 

 mean the relations of the ancestral type form to its environment), 

 we can, I think, quite conceive that there may be sufficient causes 

 operating to bring about such an effect as we see in the tbin shells 

 of H. aspersa, even after only a few short generations. 



Binney in his " Land Shells of the United States," points out 

 that Helix hortensis, a common European banded snail (only 

 recorded with us from N.Z.), has been transplanted to some of the 

 small islands on the east coast of the United States in the vicinity 

 of Cape Ann, occurring there in countless numbers. It is wortby 

 of notice that each island is inhabited by a variety peculiar to 

 itself, showing that the variety which happened to be introduced 

 there has propagated itself without a tendency to run into other 

 varieties. 



It will be noticed that we have as yet very few records of the 

 imported species, absolutely none for some of the colonies. It 

 would be worth while for those interested in the subject to help in 

 extending our knowledge by collecting and preserving examples of 

 these neglected forms for the purpose of identification and the 

 completion of a comprehensive series of records. The best plan 

 for preservation is to drown the animal in water before placing in 

 spirits. 



Good figures of the Slugs may be found in Zeitschrift fiir 

 wissenschaftliche Zoologie, 1885, plate vii., with an article on the 

 European species by Dr. Simroth, whilst full lists of synonyms 

 may be obtained from Lovell Reeve's " British Mollusks " and 

 other works. 



