SCIENCE-GOSSIP. 



161 



COLORATION AND VARIATION OF BRITISH EXTRA-MARINE 



MOLLUSCA. 



By Arthur E. Boycott. 



"THE problems of variation have of late years 

 been a subject of study with an ever-increas- 

 ing body of naturalists. People who are not 

 professional investigators of science have begun 

 to recognize that the be-all and end-all of their 

 work is not merely to collect " all the British 

 species" of a certain group. Xor yet is it to 

 discuss, with much vehemence, the question of 

 specific versus varietal rank, or whether a special 

 butterfly has really been caught in Kent. 



The bitterness of the arguments is often quite 

 disproportionate to the importance of the points 

 at issue. At the same time it is a cause of infinite 

 congratulation that we see less and less of this kind 

 of thing in the literature of British land and fresh- 

 water mollusca The number of species is so small 

 that even the 'mere collector"— I speak in no 

 disparaging sense— is compelled to turn his atten- 

 tion to the varieties which are so well marked 

 and numerous. William Bateson, in his book, 

 " Materials for the Study of Variation," says : 

 " Many of the problems of variation are pre- 

 eminently suited for investigation by simple means. 

 If we are to get further with these problems, it 

 will be, I take it, chiefly by study of the common 

 forms of life. Anyone can take part in this 

 class of work, though few do." The whole 

 question rests fundamentally on a very extensive 

 collaborative and systematised collection of facts, 

 often minute, many of them matters of simple 

 observation, requiring no special training or 

 qualities beyond those which every naturalist must 

 possess — eyes, fingers and common sense. One of 

 the most important details in the work is, what is 

 done should be accurate. To know that a certain 

 locality produces Pupa secale, for example, of a 

 larger and stouter build than some other spot, is 

 interesting, but now more or less useless, unless 

 we know how big the specimens are from each 

 place (') This entails measuring the shells if it be 



•ion of variation in size, which is, it must 

 be confessed, very tedious and laborious work, 

 especially if there U practically little or no varia- 



in a long scries. Work di 

 this way admits of general tabulation Thi 

 important part of the work in that ma:. 

 should do it simulta: general 



plan, so that all the results may from lime to time 



Elected and collectively examined The 

 present sketch is not meant by any means to be a 

 ' of the subject, the friiijje ui which 

 I tn*Ai oratm-ntt t- 



has only just been touched ; but its object will have 

 been amply fulfilled if more conchologists can be 

 persuaded by these notes to forsake the paths of 

 species-hunting, and turn their attention to the 

 more general biological problems. The variations 

 found in British terrestrial and freshwater 

 mollusca are really extremely large. We have 

 only to turn to the pages of J. W. Taylor's (*) 

 recent book to find examples: e.g., Cryptomphalus 

 aspersus may vary in weight from 0259 to more 

 than 6 - jS grammes, and Anodonta cygnca from a 

 normal of 21 grammes to as much as 159 grammes. 

 This large variation renders the group particularly 

 adapted for study, especially when we take into 

 account the fact that they show an unusual 

 plasticity in form, etc., according to differences in 

 habitat and other " attendant circumstances." 



First, then, to consider the practical methods of 

 studying variation in size. Here, naturally, the 

 most important instrument is one by which we can 

 accurately ascertain the size of our shells (-). The 

 rough method of direct application of a rule to 

 the shell is altogether too crude, except for large 

 Anodonta and such shells. The method of callipers 

 is much better, but in many cases is not susceptible 

 of sufficiently fine work. It is always difficult to 

 manipulate it with any certainty and rapidity. The 

 best instruments for the purpose are no doubt the 

 micrometer screw gauges made by the various 

 physical instrument makers, though too often 

 at somewhat exorbitant prices. These measure 

 to coi mm. as a rule, which is an accuracy 

 rather beyond what is generally required ( :l ). 

 On the other hand, the direct scale can really 

 only measure to whole millimetres, and that 

 inaccurately, which is going too far to the less 

 preferable extreme. Accurate measurement in 

 whole millimetres is not fine enough for many of 

 the smaller ICnglish species. An instrument which 

 I have used with all success, and which has given 



[onograpb of Br. I.. I', w. Moll, i. (part ii.i, pp. 7779 



(t«j5). I hi-.- given rathei 'nil references 11 gnout. In 



ncrhap unnecessarily ; l>nt I trust they will be 

 tii 1 authoritl for my facts, and as Indicat- 

 ing where a fuller treatment of tin: various points may be 

 found. 



11 itlon in lbs ibi II is mainly consldi n 'I .1 ' 



■ hi pari to study. In many \v;i\h i( I', also an Indl ■■ 



10 the in Inhabltanl rbe old method, which 



gave all noil, equally wltb the new, which 



1 1 . this Important and pro it organ, art errom oui , 



1 j ■■iHt nl view. 



■ i hardly nacosi at ■,■ it- 1 ■- to urge 



sndgrammi Innllaucl taaui 



mam . !>.«■! not I oni ■ fori 1 on thi ubjeel 



1 ■ .,1 Incbol and grammes, gralm and 



u, oi to-., ...iiii (ba ■ itraordlnary statement thai mllli- 



ilvea " would bavt l>< • a absurd." 



