1877.] The Colors of Animals and Plants. 643 
invariably yellow, and our poppies red, while many of our but- 
terflies and birds resemble each other in every spot and streak of 
color through thousands of individuals. We also find that color 
is constant in whole genera and other groups of species. The 
Genistas are all yellow, the Erythrinas all red; many genera of 
Carabide are entirely black; whole families of birds —as the 
Dendrocolaptide — are brown; while among butterflies the nu- 
merous species of Lyczna are all more or less blue, those of 
Pontia white, and those of Callidryas yellow. An extensive 
survey of the organic world thus leads us to the conclusion that 
_color is by no means so unimportant or inconstant a character 
as at first sight it appears to be; and the more we examine it 
the more convinced we shall become that it must serve some pur- 
pose in nature, and that besides charming us by its diversity and 
beauty it must be well worthy of our attentive study, and have 
many secrets to unfold to us. 
In order to group the great variety of facts relating to the 
colors of the organic world in some intelligible way, it will be 
best to consider how far the chief theories already proposed will 
account for them. One of the most obvious and most popular 
of these theories, and one which is still held, in part at least, by 
many eminent naturalists, is that- color is due to some direct 
action of the heat and light of the sun, thus at once accounting 
for the great number of brilliant birds, insects, and flowers 
which are found between the tropics. But here we must ask 
whether it is really the fact that color is more developed in trop- 
ical than in temperate climates in proportion to the whole num- 
ber of species ; and, even if we find this to be so, we have to 
Inquire whether there are not so many and such striking excep- 
tions to the rule as to indicate some other causes at work than 
the direct influence of solar light and heat. As this is a most 
‘portant question we must go into it somewhat fully. 
It is undoubtedly the case that there are an immensely greater 
number of richly-colored birds and insects in tropical than in 
temperate and cold countries; but it is by no means so certain. 
that the proportion of colored to obscure species is much or any 
greater. Naturalists and collectors well know that the majority 
_ Of tropical birds are dull colored ; and there are whole families, 
Comprising hundreds of species, not one of which exhibits a par- 
ticle of bright color. Such are the Timaliide of the eastern and 
the Dendrocolaptide of the western hemisphere. Again, many 
&roups of birds, which are universally distributed, are no more 
