1S78.] Colouration of the Organic World. 41 
darkening effect upon others, according to their different 
molecular structure. There is, for instance, little doubt but 
that the air of Persia is, as a rule, exceedingly transparent ; 
the climate is dry, mists and clouds comparatively rare, 
woodlands scanty, and the country generally open. We 
have even heard it stated that there the satellites of Jupiter 
are occasionally visible with the naked eye. Here, there- 
fore, we have doubtless a case of light in its greatest inten- 
sity ; but, according to Mr. Blanford, Persian specimens are 
generally paler than their nearest European representatives. 
Here, if light be directly concerned, its aCtion must be of a 
bleaching character ; yet we generally find in mammals, in 
birds and reptiles, as well as in inseCts, the upper surface, 
or portion most exposed to the sun, is darker than the under 
side, or than parts generally kept in the shade. An animal 
in whom the contrary arrangement prevails — -such as the 
common badger — has much of the appearance of a caricature. 
This darkening of the superior surface of animals is again 
adduced as an instance of the chromogenic power of light, 
a view to which we shall afterwards take occasion to revert. 
As regards the comparison between the tropical and the 
extra-tropical faunae the case may, perhaps, be fairly summed 
up thus : — There are certain cosmopolitan groups whose 
members, wherever found, are alike devoid of rich or bril- 
liant colouration ; there are other groups — -such as the 
Ornithoptera, the Papiliones, the Buprestidae, the Cetoni- 
adae, the trogons, humming-birds, birds of paradise, &c. — - 
which have a remarkable and hitherto-unexplained tendency 
to the development of splendid hues, and which, if not ex- 
clusively tropical, have their head-quarters and produce 
their largest representatives within the torrid zone. Other 
groups, again, attain their greatest splendour beyond the 
tropics, as, e.g., the ducks, the pheasants, and among inserts 
the ground-beetles or Carabidae. It has, indeed, been sug- 
gested that if the colder regions of the earth are now 
inferior to the tropical districts in the beauty of their fauna, 
the cause may be sought in the ravages of the Glacial 
epoch. If the most magnificent species were forest-dwellers, 
as we now find it to be the case in warm climates, their 
destruction would be almost inevitably involved in the deso- 
lation of their haunts and the annihilation of their food. 
Perhaps, too, the very splendour of such supposed forms 
would render them more conspicuous to their enemies, and 
thus accelerate their extirpation. All such speculations, 
however, are little more than conjectural. We conclude 
indeed, judging from the fossil remains of inseCts discovered 
