84 NATURAL SCIENCE [August 



The student of slugs obviously cannot be a mere conchologist, 

 but one of two temptations may beset him according as he is or is 

 not an anatomist. In the latter case, he may be led to attach 

 undue importance in descriptive work to colour variations, occasion- 

 ally making one species out of two, but more often splitting up one 

 into several, and possibly naming ' varieties/ which may be based 

 merely upon such phases of coloration as succeed one another 

 during the life-history of a single individual. The anatomist, on 

 the other hand, may be inclined to consider as diagnostic of species 

 points of internal difference so small as to suggest that the describer's 

 wish was father to his thought. And this is especially the case 

 when little or no outward differentiation is obvious. Of course, in 

 these days when ' physiological species ' are recognised, there is 

 nothing inherently improbable in the assumption that there are 

 forms alike without yet not within, provided that some additional 

 evidence is forthcoming as to varying habits or development. It 

 seems reasonable, however, to expect something of the latter sort to 

 be brought forward before we are asked to recognise ' anatomical 

 species ' as good ones. 



Mr Collinge is an anatomist, and in his paper " On some 

 European Slugs of the genus Avion" (Proc. Zool. Soe., 1897, 

 pp. 439-450, pis. xxix.-xxxi.) he brings forward evidence as to 

 " the constancy of anatomical characters " so far as the genital 

 organs are concerned, these being the parts in which specific differ- 

 ences are mainly sought. The testimony is based upon the small 

 number of variations found in a large series of specimens dissected, 

 and belonging to the same two species of Avion. That the form of 

 generative organs in the particular genus considered presents but 

 little modification throughout the species is a well-known fact, and, 

 even if this were not so, a comparison between fig. 3 of A. subfuscus 

 and fig. 12 of A. hovtensis would suffice to show it. To bring- 

 forward this ' constancy ' in Avion as militating against Cockerell 

 and Larkin's belief in the specific identity of several forms of Vevoni- 

 cella, which differ anatomically, is but lost labour. So far as Avion 

 itself is concerned, the small number of slight variations noted by 

 Mr Collinge (i.e., 26 out of 1223) may serve as an excuse for 

 increasing the number of species when more marked differences are 

 from time to time detected. 



Mr Collinge, indeed, in the paper under discussion, reserves his 

 Avion hovtensis, var. caevuleus, which differs more markedly from 

 the typical hovtensis, and raises it to specific rank. We note that 

 A. hovtensis is figured with but one vestibule, while caevuleus has 

 two : two, however, are shown in an unpublished drawing of the 

 former by the late Charles Ashford now before us, while Mr 

 Collinge's fig. 1 2 contradicts his own statement, that in no species of 



