excitability. In many groups in which this superabundant vitality is at
a maximum, the development of dermal appendages and brilliant colours
has gone on increasing till it has resulted in a great diversity between
the sexes; and in most of these cases there is evidence to show that
natural selection has caused the female to retain the primitive and more
sober colours of the group for purposes of protection.

_Concluding Remarks._

The general principles of colour development now sketched out enable us
to give some rational explanation of the wonderful amount of brilliant
colour which occurs among tropical animals. Looking on colour as a
normal product of organisation, which has either been allowed free play,
or has been checked and modified for the benefit of the species, we can
see at once that the luxuriant and perennial vegetation of the tropics,
by affording much more constant means of concealment, has rendered
brilliant colour less hurtful there than in the temperate and colder
regions. Again, this perennial vegetation supplies abundance of both
vegetable and insect food throughout the year, and thus a greater
abundance and greater variety of the forms of life are rendered
possible, than where recurrent seasons of cold and scarcity reduce the
possibilities of life to a minimum. Geology furnishes us with another
reason, in the fact, that throughout the tertiary period tropical
conditions prevailed far into the temperate regions, so that the
possibilities of colour development were still greater than they are at
the present time. The tropics, therefore, present to us the results of
animal development in a much larger area and under more favourable
conditions than prevail to-day. We see in them samples of the
productions of an earlier and a better world, from an animal point of
view; and this probably gives a greater variety and a finer display of
colour than would have been produced, had conditions always been what
they are now. The temperate zones, on the other hand, have recently
suffered the effects of a glacial period of extreme severity, with the
result that almost the only gay coloured birds they now possess are
summer visitors from tropical or sub-tropical lands. It is to the
unbroken and almost unchecked course of development from remote
geological times that has prevailed in the tropics, favoured by abundant
food and perennial shelter, that we owe such superb developments as the
frills and crests and jew