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 fcetal 

 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. 



