It ON ANIMAL LIFE a & 
the beauty of animals and plants are not only 
thus a delight to the eye, but afford us also 
some of the most interesting problems in 
Natural History.. Some probably are not 
in themselves of any direct advantage. 
The brilliant mother-of-pearl of certain shells, 
which during life is completely hidden, 
the rich colours of some internal organs of 
animals, are not perhaps of any direct 
benefit, but are incidental, like the rich and 
_ brilliant hues of many minerals and precious 
stones. 
But although this may be true, I believe 
that most of these colours are now of some 
advantage. “The black back and _ silvery 
belly of fishes’’ have been recently referred to 
_by a distinguished naturalist as being obvi- 
ously of no direct benefit. I should on 
the contrary have quoted this case as one 
where the advantage was obvious. The dark 
back renders the fish less conspicuous to an 
eye looking down into the water; while the 
white under-surface makes them less visible 
from below. The animals of the desert are 
sand-coloured ; those of the Arctic regions are 
