VARIATION IN COLOUR. 
93 
that such conditions will assist in the elimination of colouring 
matter, and tend towards the dull white and bandless forms especially 
characteristic of arid and desert districts. 
Leucochroism (XevKoi white and colour) has been defined as 
that state wherein the darker shades of the ground colouring or the 
markings thereon are diminished in intensity, in extent, or both 
combined, owing to the diffusion of a paler shade of colour, or to the 
darker markings assuming a paler tint than is usual in typical in- 
dividuals. This state may he considered as more or less intermediate 
between the ordinary typical condition and the albine form, and the 
illustrations of this variation are very numerous amongst mollusks, 
the bandless varieties or the pale or thin banded forms of Ileli.r 
nemoralh and the dull white bandless varieties of Helix virgatd may 
be quoted as familiar instances (see pi. ii., fig. 4). One of the 
causes of this state has been assumed to be exposure to dryness and 
warmth, circumstances which M. Strobel has shown to induce this 
state in Helix virgatd, and the accuracy of this opinion is confinned 
by the knowledge that such forms are the prevalent ones in desert 
regions, their colouring reflecting the heat to which they arc exposed, 
and therefore preventing the drying-up of the natural moisture of the 
animal. Mr. Dali thinks this leucochroic condition may be due to a 
less fluent secretion of the animal products, which are the chief 
components of the glistening epidermis of shells native to moister 
regions. This and other variations may however also arise as a con- 
sei^uence of the protection afforded by resemlilance to their sur- 
roundings, as may to a certain extent be the case with the variety 
dlbescens of Cyclostomd elegdns found by Mr. Brockton Tondin, upon 
the white chalk cliffs at Lulworth. 
Melanochroism (geX-ii-v black and colour) is the opposite 
tendency to leucochroism, as it expresses the increase of darker shades, 
either of the ground tint or markings, at the expense of the lighter 
shades of colour, and is a tendency towards true melanism, being the 
intermediate stage between the typical individual and the specimen 
with black ground colour or markings. The term Phteism (f/)atos 
dusky), which applies to dusky specimens, is not so precise in its 
application and may refer to a case of leucochroism or melanochroism 
according as the affected individual is more darkly or lightly pig- 
mented than the typical form, without distingtiishing the mode by 
