feOYCOTT : THE HABITATS OF fkESHWATtk MOLLtJSCA. 25I 



they are widely dispersed and far from common, that is they are rare 

 rather than local. What is the particular kind of liabitat which they 

 require ? Is Neritina dominated by the oxygen supply, as attempts 

 to keep it in confinement would suggest ? Is the occurrence of mar- 

 garitijera conditioned by the relative absence of lime ? It is evidently 

 easier to ask questions than to answer them. 



The solution of these problems may be approached along two lines. 

 First, both in time and importance, we want to clearly know the facts. 

 What is true of one locality does not necessarily hold in another, and 

 an adequate experience is beyond the range of the most leisured 

 observer ; co-operation is essential. Definite lists of species found in 

 defined loci will soon give some of the general rules which we seek 

 to find, as, for example, what species commonly live together. The 

 " well known association of Fl. spirorbis and F. hypnorum " illustrates 

 the need for precise data. At present the idea rests on impressions ; 

 the presence or absence of a real association can be based only on a 

 knowledge of the numbers of loci — (i), with F/. spirorbis, (2), with 

 F. /ijpfwrum, (3), with both, (4), with neither. Systematic surveys 

 of districts in which all water units are examined would be still more 

 valuable. In trying to define what sorts of habitats are favoured by 

 what sorts of snails, it is difficult to say just what other data should 

 be taken ; experience alone will indicate. It is impossible to observe 

 everything, but as a preliminary a statement of (a), the general 

 physical nature of the locus, size, depth, character of water, still, 

 permanently running, temporarily running, etc., (b), the more obvious 

 plants present, and (c), any animals (^e.g. Gamtnarus\ alg?e, etc., 

 which the observer happens to be able to identify, will probably 

 suffice to give a tolerably good idea of the general facies and charac- 

 teristics. Especially to be avoided is that curse of the human mind 

 — to pay attention to the rare and exceptional to the exclusion of the 

 common and normal ; we must discover the everyday rules before we 

 can hope to explain the exceptions. 



Secondly, in supplement and suggested by these observations, ex- 

 perimentation is wanted in the field and under artificial conditions. 

 The transplantation of species from one natural locus to another 

 within the same district will not, I hope, offend the most sensitive 

 geographer ; experiments on the capacity of the different species to 

 live in various artificial environments will throw light on their relation 

 to aeration, muddiness, food, calcium salts, decaying vegetation, other 

 animals and the like. By this means alone can the various factors 

 be dissected. We must not, however, be too quick to assume that 

 our experiments will give answers which can be translated directly to 

 field experience. The bits of seashore and heath which live in 



