ON COLOURING MATTERS OF VARIOUS ANIMALS. 19 
colours produced by absorption spectra, such as turacin, 
have a tint which strikes the eye at once as remarkable 
and peculiar, yet it is impossible to tell beforehand which 
colouring matters will yield absorption bands in their spectra 
and which will fail to do so. 
It seems improbable that the eyes of other animals are 
more perfect as spectroscopes than our own, and hence we 
are at a loss for an explanation on grounds of direct benefit 
to the species of the existence of the peculiar complex pig 
ment in it. That the majority of species of Antedon should 
have vivid colouring matters of a simple character and that 
few or one only should be dyed by a very complex one is a 
remarkable fact, and it seems only possible to say in regard 
to such facts that the formation of the particular pigment 
in the animal is accidental, i. e. no more to be explained than 
such facts as that sulphate of copper is blue. 
A certain organic compound becomes formed in the animal 
or plant in course of evolution, either as a directly serviceable 
tissue-forming element or gland component, or possibly as an 
inert and almost excretory product. And this compound has a 
complex absorptive action on light. In some animals and plants 
the coloured compound is turned to account by natural selec¬ 
tion, 1 increased in quantity and distributed in various ways, 
either for sexual adornment, concealment, or possibly in such 
cases as Actinia for the attraction of prey; in others it remains 
unused. In some instances a colouring matter may exist in an 
animal or group as a rudiment, having lost a sexual or other use 
which it had in the ancestors of the animal in question, but 
having persisted. No doubt this is the case with the colour¬ 
ing matters of many deep-sea animals. In some cases, again, 
a complex substance, produced by evolution for strictly 
physiological purposes, and happening to have a bright 
colour, may be turned to further advantage by some animals 
possessing it for beneficial external adornment. This would 
seem to be the case with hremoglobin, the redness of which, 
considered as to the colour only, has no use in the majority of 
animals, and is indeed mostly concealed in utter darkness ; 
but in some instances, as in the cock’s comb and in the faces 
of the white races of man, is turned to account for sexual 
adornment. It is quite possible that in suoh instances as Pen- 
tacrinus the very abundant colouring matter (Pentacrinin) may 
have some important physiological function as yet unknown. 
It is remarkable that in animals coloured by most widely 
different colouring matters albinism should occur in certain 
numbers of individuals of a species. 
' Notably the case of chlorophyll in green plants. 
