Clare Island Survey — Land and Fresh-water Mollusea £3 1 1 



This makes one consider for what reason do we study the range of a 

 particular species ? Everyone must answer this question after his own 

 fashion and according to his own view of the subject ; nevertheless, most will 

 agree that the final aim of geographical conehologists is to determine the 

 natural range of a species before man's influence acted upon it. If this be true, 

 the publication of " garden " records is the greatest barrier which stands 

 between us and our goal. 



Gerrard, writing in the sixteenth century, says of a certain plant:— "It 

 groweth upon the mountaines of Germanie : we have great quantitie of it in 

 our London gardens." But although botanists have long ago come to 

 distinguish between " casual," " native," and " naturalized " plants, few 

 conehologists when publishing their local lists have stated whether any of 

 the species included in these are confined to gardens or cultivated ground. In 

 a part of the country such as West Mayo, where man's influence has been so 

 little felt, it is a simpler matter in most cases to separate the natives from 

 the introduced species than in the surroundings of London, Manchester, or 

 other large towns. Because a mollusk is found in a garden it does not 

 necessarily follow that it has been imported by artificial means ; yet such a 

 record is quite useless for the purpose of geographical distribution. Thus, in 

 the districts included in the Clare Island Survey, Limax flavus, Milax sowerbyi, 

 Arion hortensis, and Hygromia rufeseens, having been taken by me only in the 

 neighbourhood of habitations, I do not consider that up to the present they can 

 be considered "native" in West Mayo. On the other hand, we cannot say that 

 they are not " native " in Ireland, nor can we say that they are not " found " 

 in West Mayo. How, then, are we to record their standing ? The formula 

 proposed by R. LI. Praeger in his report on Clare Island Flowering Plants 

 (Part 10 of this series) solves, so far as I can see, this difficult problem. 

 Praeger in this adopts Dunn's definition of a native (S. T. Dunn: Alien 

 Flora of Britain, pp. 9-10). A native species is defined as one living 

 in a natural habitat, which it has reached by natural means from an 

 uncontaminated source. This is designated NNN, while a species, in an 

 artificial habitat, to which it has been brought by artificial means from a 

 contaminated source, is shown by ***. Between these two we have six sets 

 of conditions, and by these eight combinations of N and *, the standing of 

 any species may be recorded, according to our own ideas on the subject. In 

 the table at the end of this report I have given the standing of each species, as 

 it appears to me, by means of the above formula. 



Were it possible to do this with every record of every species for every 

 division of Ireland, the NNS distribution of some shells, such as Helix aspersa, 

 Milax sowerbyi, Limax flavus, and Hygromia rufeseens, would differ essentially 



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