230 COCKERELL AND COLLINGE : CHECK-LIST OF SLUGS. 



of numerous malacologists on the changes effected by habitat, 

 climate, food, &c, are known to all. Important as these various, 

 variations may be — I must confess I cannot attach the importance 

 to them that some have done — they are, from the variability of their 

 nature, unsuitable for generic or specific distinction, except as 

 secondary characters. Recourse must, therefore, be made to the 

 anatomy. In the form of the various organs we find a permanent 

 and well-marked difference between one genera or species and 

 another. It must be remembered that there are slight variations in 

 the form, colour, &c, of the various organs according to age, 

 season, &c, so that a reasonable limit must be allowed in which a 

 species may vary. The anatomical differences which distinguish 

 Arion from 7estacella, or Testacella from Veronicella, are at once 

 appreciable. Not only is this so, but, generally speaking, an 

 acquaintance of any duration will enable the student in most cases 

 to readily distinguish between one species and another from the 

 morphology of the generative organs alone ; where these are 

 undeveloped or at all doubtful, the nervous and digestive systems- 

 are almost as serviceable. Seeing, then, that the external features 

 are liable to such change, and that the internal are much more 

 constant, I prefer to accept the latter, and upon these build up a 

 rational basis for a system of classification. The old system of 

 systematic zoology has had its day, and as a warning (and in 

 evidence of its worthlessness) leaves us volumes of synonyms to 

 almost every species, not to mention any of the graver errors it has 

 been the source of. With the advance of morphological investiga- 

 tion, the student, instead of revelling in the multiplication of so- 

 called species and varieties (to me Bourguignat and Westerlund are 

 a terrible warning) endeavours to classify upon some scientific basis, 

 and then to show the genetic or specific relationship between one 

 genera or species and another. A study based on such lines rises to 

 one of importance and value ; on the old, resembles a schoolboy's 

 attempt at a system of ethics — in short, becomes a farce. 



Until Professor Cockerell describes and figures the anatomical 

 differences in his species of slugs, I cannot accept them as valid. 

 I do not say that they are not so, as many seem to be very distinct, 

 judging from the external features, &c., but until I see structural 

 differences — not mere variations in the breadth or colour of some 

 single organ — differences which mark them off in the majority of 

 individuals from their nearest known ally, I shall regard them as 

 doubtful. 



If we allow a species (or variety) to vary within a certain limit, 

 there is no need for any such things as subspecies (or subvarieties) 



