33 
The Action of Light upon the [January, 
vitality is at a maximum.” But we are not aware that either 
Mr. Wallace or any one else has fully grasped the principle 
laid down above, or traced its numerous applications, aesthetic 
as well as biological. 
But among the “ pigment-colours ” there is a very great 
diversity in permanence due to the nature of the colours 
themselves, or to that of the tissues in which they inhere. 
Dr. Hagen divides such colours into epidermal, placed in 
hair, in feathers, and in the chitinic exo-skeleton of inserts, 
and hypodermal, situate in the softer internal layers of the 
skin. That the latter are the more easily affedted by any 
external influence is natural. 
Alterations and degradations of colour similar to those 
above-mentioned may indeed, under certain circumstances, 
be produced even in the absence of light. But we have 
direct experimental evidence to show that, other things 
being equal, animal matters retain their colours most com- 
pletely in the absence of light, and fade the more rapidly in 
proportion to the intensity of the illumination to which they 
are exposed. Hence we are compelled to recognise light as 
a destroyer of animal colouration. 
But light is generally regarded not merely as a colour- 
destroyer, but as a colour-producer, and it is with this its 
supposed fundfion that we have now to deal. Those who 
take here the affirmative view rely mainly on two fadts, or 
supposed fadts, to which we have already briefly referred, — the 
higher colouration and the superior brilliance of the tropical 
fauna, and the sombre hues of nodturnal and subterranean 
beings. At these fadts we must look, and seek to ascertain 
their meaning. We must of course admit that Europe pro- 
duces no humming-birds or trogons, no Belionotce or Pachy- 
vhynchi ; but we must also remember that the total number 
of species of birds, of reptiles, and of insedts found, say in 
South America, is far greater than the sum total existing in 
Britain or on the European continent. Hence, even if the 
tendency to produce a gay colouration were equal in either 
case, the probability is that South America would be the 
richer in gorgeous species. Again, travellers who visit 
tropical countries not unnaturally seledt the most showy 
forms, and their collections are therefore not a fair average. 
Naturalists, such as Mr. Wallace, who have taken the 
trouble to examine closely, find that even in New Guinea, 
Borneo, or Brazil dull-looking species exist in numbers. 
Had we catalogues of the insedts of such countries as com- 
plete as those we possess for Britain, France, or Germany, 
our views as to the general character of a tropical fauna 
