26 JOURNAL OF CONCHOLOGY, VOL. 12, NO. I, JANUARY, I907. 



species ? If we read over Darwin's remarks carefully we find that 

 in several instances he is doubtful about the specific distinction of 

 some forms that had been separated by specialists and that he points 

 out in other cases that the differences are varietal. He was so struck 

 by the resemblance to the continental types that he evidently 

 thought it advisable for this resemblance to be recognized by a 

 varietal appellation only. 



Let me give a simple illustration of the difficulty. Lundy Island 

 has two beetles peculiar to its small area and found nowhere else. 

 These differ from their English congeners mainly in colour; in the 

 one case it is the colour of the legs that is distinctive and in the other 

 that of the wing-covers or elytra. One is accounted a species and 

 the other a variety, although no valid reason can be given for the 

 differentiation. At first it might be thought that minor differences 

 should be considered varietal and more strongly marked ones specific 

 in their value. But minor and major are at best merely comparative 

 and who will be able to settle what are major and what minor in 

 a particular case ? 



As an illustration of the great range of variation that may occur 

 in a single species, I here exhibit a few forms of the common whelk, 

 Buccinum undatum L., all of them adult specimens. These show 

 great variation in every character of the shell, except the spiral 

 striation. Size, thickness or the shell, strength of the longitu- 

 dinal ribs, and proportionate length of the spire all show a far 

 greater degree of variation than would often be observed between 

 different species. And yet they are not in any way abnormal 

 specimens, but average examples oi well known racial varieties that 

 occur in the comparatively small area of British waters. Many of 

 us remember how in this room the Rev. Canon Norman exhibited 

 his fine collection of Buccina and how, after pointing out one 

 after another of several so-called species, he said "They are all 

 one." The more we study the known variation of certain species the 

 more hopeless do we find it to assign definite limits to the range of 

 possible variation. Consequently we cannot say that two forms 

 belong to separate species because they are unlike. 



Selected series of Clausilia bidentata are also exhibited to illustrate 

 the fact that in this species there is not even one constant character. 

 Size, shape, texture, colour, number of whorls, sculpture, and the 

 plication of the mouth all show great variability. 



There is yet another fact to be considered and again we will take 

 an exemplary case. Two of our British Mactras, Spisiila solida and 

 6". elliptica usually inhabit difi'erent zones and are quite distinct, the 

 first living on the shore or in very shallow water and the second 

 being found in deeper water. No intermediate forms are ordinarily 



