264 MR. G. C. ROBSON ON THE 



probably Gonditionecl by the amount of secretion which is nsnally 

 subject to periodic variation. 



The late B. F. Cummings and the present author put forward 

 a suggestion ("X" 1914) that internal structures would probably 

 turn out to be better guides for ordinal and generic characters, 

 while the external stiuctures register specific and varietal 

 differences more readily. There would now be grovinds for 

 considering that the internal anatomy is neither more nor less 

 variable than the external structures. 



The small amount of correlated variation in these forms, and 

 the irregular fashion in which individuals of a variety agree or 

 differ from those of a closely allied variety, make it very difficult 

 to distinguish the " species " and " varieties " of current taxonomy 

 in this case. The author has come to the same conclusion as 

 Pilsbry (1912), that "a philosophic method of dealing with intra- 

 specific differentiation is one of the greatest needs of systematic 

 zoology." "Species" and "variety" appear to the author, and 

 also apparently to Dr. Pilsbry, to be an association of forms 

 which in their mutual relationship may be singularly unstable 

 and loosely associated. It is not merely a question of the 

 extreme linear or graduated variability with which all naturalists 

 are familiar. In that case species and varieties are equivalent 

 for example to the divisions of time into hours and minutes as 

 convenient, if arbitrary, units. In the present case, however, 

 the large amount of uncorrelated variation between individuals 

 of a "variety" leaves a very strong suspicion in the mind that 

 along with linear (or giaduated) variability there may exist a 

 radial * tendency of association which cannot be expressed in 

 arbitrary subdivisions. If this impression is substantiated by 

 other evidence it will remain to discover which of the two ten- 

 dencies corresponds more closely with the results of cytology and 

 genetics. The author is indebted to Professor Duerden for per- 

 mission to state that certain conclusions ariived at by him in 

 the coui'se of work upon the Ostrich are in line "with the facts 

 recorded above. 



lY. On the Classification of the Achat fNi n^e k^d 

 THE Position of the Genus Cochlitoma. 



Pilsbry (1904) has taken the shell as his basis for arranging the 

 twelve genera of Achatininse. and his first subdivision into two 

 main groups depends upon the presence or absence of sculpture 

 on the embryonic whorls. We cannot, however, disregard the 

 fact that examination of a number of examples in the British 

 Museum collection has made it clear that the sculpture of the 

 embryonic whorls of these forms differs in individuals of the same 



* The antithesis between linear and radial variation used in this connection maj- 

 be criticized as vague. It is difficult, however, to find two terms whicli express 

 adequately the fact that an individual of a "species" placed in a series presumed 

 to express descent maj^, however, exhibit characters which qualify it for a place in 

 another series not necessarily expressing the same line of descent. 



