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. Soc, 1897, 
pp. 439-450, pls. 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 Arion. 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. swbfuscus 
and fig. 12 of A. hortensis 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 Veroni- 
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¢, 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 
Arion hortensis, var. cacruleus, which differs more markedly from 
the typical hortensis, and raises it to specific rank. We note that 
A, hortensis is figured with but one vestibule, while caeruleus 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. 12 contradicts his own statement, that in no species of 
