166 GENUS ARION. 
Variation.—In Arion the pigmentation has resolved itself into two 
chief colours : one dusky or black producing the markings, and the other 
various shades of red, both of which in the larger and more variable 
species by their development or degeneration quickly show, especially 
during the growth period, the effects of varied environment, mirroring 
back in the slug’s body the effect of climate, etc. Cold and moisture 
being favourable to the development of the dusky pigment and warmth and 
dryness to the red. 
The darker varieties are found most numerously in the northern regions 
and in mountainous districts, and this melanic tendency is correlated with 
a stouter and coarser condition of the skin, and by a bolder tuberculation, 
while the brighter coloured forms are probably the outcome of a dry and 
warm environment, and though these factors are not the only ones that 
foster a brilliancy of colouring or the reverse, yet they undoubtedly have a 
very marked influence thereon. 
The largest and most advanced species are those most actively engaged 
in the elaboration of new forms, but it is amongst the smaller and more 
primitive species that convergence is most evident, and which display most 
strikingly their common relationship to an ancestral form. 
Geographical Distribution.—The Avions are European in origin and 
distribution, affecting more particularly the western area; members of 
the group have, however, spread into Siberia, North Africa, Azores, 
Madeira, etc., and through the agency of commerce have secured a footing 
in North America and other places. 
It has, however, been erroneously supposed by Bourguignat and other 
able scientists that Avion, as well as other genera, originated on the 
Central Asian plateau, and spread into Europe by way of the great 
northern coniferous belt, or along the mountain chains of Central Europe, 
while other modern authors believe the Iberian peninsula or the lost 
Atlantis to be the true birthplace of the group. 
Fic. 184.—A Malacological Laboratory. 
Mr. W. E. Collinge at the University, Birmingham. 
