BOYCOTT: r.OCAL VARIATION OF CI.AUSIT.IA BIOENTATA. It 



lieiistome are considered ; these have a certain length or altitude and 

 a certain width or diameter, as to which the configuration of the 

 sliell liardly allows of any ambiguity. The measurements are con- 

 veniently made with a small sliding gauge such as is used by watch- 

 makers ; the particular one which I use was introduced to me by 

 my mentor in natural knowledge, E. W. Bowell, and has been a 

 consistent friend for more years than I care to remember. With a 

 vernier it reads to o'l m.m., and repeated measurements of the same 

 specimens indicate that the figures are correct to something less than 

 o"i5 m.m. Any such gauge will do so long as it slides; a screw 

 gauge breaks the shells. 



Our question then would be easy enough to answer if all the speci- 

 mens from any one locus were of the same size. One would simply 

 have to catch a snail in each place and measure it. That there is 

 no such convenient uniformity, however, is one of those facts so 

 obvious to every collector that few have thought it worth much con- 

 sideration ; but it is from this variability which is inherent in all the 

 parts of live things that many of our difficulties arise. 



(i). The most assiduous collecting will not yield us every speci- 

 men from the locus under examination.' The specimens which 

 come to measurement will therefore be a sample of what are there. 

 This sample must be, as far as may be, truly representative of the 

 whole snail population, and the three considerations which are 

 immediately germane are 



(a) that the whole sample should be collected at the same time,' 

 or at any rate at times sufficiently near together to exclude the 

 supervention of a new generation : 



(/3) that the collection should be without bias, that is that every 

 specimen seen should be taken ; for fear of missing small indi- 

 viduals collect also those not fully grown, and sort them out 

 afterwards, and in every way take care to avoid selection of any 

 kind : 



(y) that the number taken should be large, a condition which 

 fortunately defies definition. Bearing in mind that the gain of 

 accuracy in the result is proportional only to the square root of the 

 number examined and, which I particular desire to emphasize, that 

 this method of enquiry is an amusing method of practical field- 

 work which is capable of yielding much new knov/ledge, I would 

 suggest IOC as a good number to aim at. Less than 20 is not 



1 Most collectors will, 1 think, agree that the snails we catch are practically only those who 

 are looking for it. Repeated collecting over the same tiny area shows how small a proportion 

 we acquire on any one occasion. 



2 If we are considering topographical variation, just as it must be taken at the same place 

 if we investigate temporal variation. 



