Abstract

UNTIL recently, the criteria of zoological and botanical relationships were, in the main, qualities of external or internal structure, or of functional behaviour. The possibly more intimate and certainly less obvious resemblances and differences between different animal types, in the chemical make-up of their tissues and body fluids and in their metabolic habits, have only recently begun to come within the sphere of established knowledge. It is as a direct result of the new methods which have been developed in biochemistry, particularly as an outcome of increased accuracy in dealing analytically with very small quantities of material, that information has accumulated in the last few years regarding these matters, throwing a flood of light on the ways in which the physical and chemical problems presented to different types of living organisms at various stages in their evolutionary history have been overcome.