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 Avill 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 accoirnt by natural selec- 

 tion,i 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 liad 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 haemoglobin, 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 such 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. 



