38 
COLOUR IN NATURE 
CHAP. 
former, in the current phrase, display more differ¬ 
entiation and more integration. Differentiation 
displays itself most obviously in an increasing 
complexity of parts, but integration is shown in 
the increasingly perfect physiological relation of 
parts. In the evolution of organisms an increasingly 
rapid elimination of waste must have been a factor 
which made for progress ; no machine can work well 
if it is choked with its own waste. Waste products 
can therefore be employed as colouring agents only 
under unusual conditions or in organisms at a rela¬ 
tively low level in evolution. Thus we see that the 
comparative study of pigments must yield important 
contributions to comparative physiology in general. 
In this connection we may notice the interest¬ 
ing fact that, in discussing the evolution of the foetal 
membranes in Vertebrates, Dr. Richard Semon con¬ 
nects the appearance of the allantois (the “ hyper¬ 
trophied urinary bladder ”) with the acquisition of the 
metanephros, a more efficient excretory organ than 
the simpler mesonephros. It is certain that the 
blood in the lower Vertebrates contains a far larger 
amount of nitrogenous waste than that of the higher. 
We shall see later that as we ascend in the scale in 
Vertebrates we have a corresponding diminution in 
the amount of nitrogenous waste products deposited 
in the skin. Now our acquaintance with the 
physiology of the Invertebrates is exceedingly 
limited, and it may well be that a knowledge of 
their pigments may be of much help in deciding 
general questions as to the phylogenetic position 
and the relative importance of structures connected 
with the excretion of waste. 
