1878.] Colouration of the Organic World. 51 
It may perhaps be thought that in an inquiry into the 
influence of light upon the colouration of animals a consi- 
deration of their diet, or of their peculiar secretions and 
excretions, is out of place. But whether solar radiations, 
or local atmospheric influences, or the need of protection 
take a greater or smaller share in the development of colour, 
there must be essential differences in the material upon 
which these causes adt. Mammals are exposed to the same 
climatic influences as birds and insedts, and are likewise 
exposed to dangers which they might escape by a coloura- 
tion favourable to concealment. Their hair is, chemically 
considered, a material no less suitable for the display of gay 
and brilliant colours than are the feathers of birds, the 
scales of serpents, or the chitinous coating of inseCts ; yet 
neither in lustre, in varying play of colour, nor in delicacy 
and elaborateness of design, do they make even the faintest 
approach to a rivalry with these groups of animals. There 
must therefore be an internal source of colouration, not 
everywhere present, upon which external influences may 
readt. 
Mr. Wallace, whilst rejecting the light-theory, brings 
forward certain principles which he considers throw a light 
upon the phenomena of colour in organic nature. Whilst 
demurring to the common conclusion that tropical light 
and heat are the cause of colour, he fully recognises the 
general fadt that “ all the more intense and gorgeous tints 
are manifested by the animal life of the tropics, whilst in 
some groups, such as butterflies and birds, there is a marked 
preponderance of highly-coloured species.” This pheno- 
menon he ascribes to a variety of causes, some of which yet 
remain to be discovered. The foremost place is given to 
the following consideration : — “ The luxuriant vegetation of 
the tropics throughout the entire year affords so much con- 
cealment that colour may there be safely developed to a 
much greater extent than in climates where the trees are 
bare in winter, during which season the struggle for existence 
is most severe, and even the slightest disadvantage may 
prove fatal.” Fully admitting the force of this consideration 
in the case of birds, we must yet, with all the deference due 
to so eminent an authority as Mr. Wallace, point out that 
it can have very little moment as regards insedts which 
during the winter are in a dormant condition, as larvae or 
violet liquid which exudes from Timavcha lezvigata, an insedt feeding upon 
bedstraw, a plant of the madder tribe, has ever been examined for alizarin or 
purpurin. 
