COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 299 
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 con¬ 
cealment, 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 geo¬ 
logical 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 jewelled shields of 
the humming-birds, the golden plumes of the birds of paradise, 
and the resplendent train of the peacock. This last exhibits to 
us the culmination of that marvel and mystery of animal colour 
which is so well expressed by a poet-artist in the following 
