90 
VARIATION IN COLOUR. 
the requi.site colouring matter, but perliaps iii some ca.ses from an 
actual deticieiicy of pigment-forming material. 
^Uhinisni may he a more or le.ss partial phenomeiiou, affecting only 
a certain set or sets of organs, ami not modifying others, or may 
embrace the whole animal, thus the shell of a mollusk may be destitute 
of colouring or pigmentation and therefore an albino, without the 
animal itself being necessarily albine. In fact, instances are number- 
less in which a stronglyi)igmented animal has borne an albine or colour- 
less shell, and the i)ossibility of this is easily appreciated when it is seen 
that the albinism of the shell solely depends upon the selective working 
of the glands of the mantle margin, which are extremely susceptible 
to different intluences, as is shown by the fact that one can hardly 
take up a .specimen of certain of our laud shells without noting the 
variability in the intensity of the colouring of the ground tint at 
different periods of growth, this being even more strongly e.xhibited 
by the spiral banding, which is freipiently found to be totally destitute 
of colouring matter at the commencement of a periodic growth, often 
acquiring the normal depth of colouring by slow degrees only and as 
the termination of the growth period is api)roached. 
'I’rue albine .shells are (piite deficient of coloured pigment and may 
have an opa(pie or a translucent character depending upon the disposi- 
tion of the constituent particles forming the shell, as to whether they 
reflect the light rays or allow their passage through the shell substance, 
and must not, as is sometimes the case, be confused with bandle.ss 
specimens of normally banded species wbicb may happen to possess 
a whiti.sh ground colour, and are therefore merely ordinary individuals 
in which the banding is suppressed; this difference is well illustrated 
by the translucent-banded form of llelt.v virgdtd and the variety 
(ilhicmis, the first being true albinos, while the latter are only ordinary 
individuals in which the banding is deficient (.see pi. ii., figs. 4, 5). 
Pai'allel cases are furnished by other species, Ilel'u' 2 >isan(i,iox instance, 
showing the two forms in the varieties alba and albtda re.spectively. 
The occasionally different colouring of the nucleus of the .shells not 
truly albine is sometimes relied upon as a means to readily distinguish 
the true albinos, but this test is not an infivllible guide. An albine 
shell, when borne by a i)igmented mollusk, is e.s.sentially similar to any 
white-furred or white-feathered animal or bird, as tbe shell, like furor 
feathers, is an e.ssentially cuticular structure (piite outside the animal, 
and, though organically connected, it has been satisfactorily demon- 
