VIS MEDICATRIX NATURE 641 



story. Is there not meaning in the long-drawn-out but 

 indubitably progressive evolution of the nervous system, in 

 the increasing elaboration of behaviour, in the gradual eman- 

 cipation of the psyche ? The bird is more of an agent than 

 the worm more of a free agent; and the world has greater 

 value to the bird than to the worm. Some simple creatures 

 have only one answer to every question ; but how complex 

 is the life of the ant on the instinctive line of evolution, 

 and of the dog on the intelligent line. Since the beginning 

 of life there has been a growing appreciation and mastery 

 of the world. Is it going to stop ? 



Perhaps no one has yet fully appreciated what may be 

 called the principle of conservation in evolution. In a very 

 literal sense, the higher animals are heirs of all the ages. 

 Let us explain. Organisms have evolved by a trial-and- 

 error method; they experiment organically, instinctively, and 

 intelligently; above all, perhaps, in the mysterious ante- 

 natal life of the germ-cells they experiment in self-expres- 

 sion just as water vapour does in snowflakes, but far more 

 subtly. What are called variations and mutations in biologi- 

 cal language are the organism's experiments in self-expres- 

 sion, and these are the raw materials of progress. 



But, while the organism is ever making tentative sugges- 

 tions and searching its environment with its tendrils, it is 

 also remarkably conservative. It proves all things, but the 

 other side is, that it holds fast that which is good. Great 

 gains once made are not held lightly. Species become ex- 

 tinct and races perish, but important organic inventions are 

 carried on by some collateral lineage. It was probably some 

 ribbon-worm that first manufactured haemoglobin the all- 

 important, oxygen-capturing red pigment of the blood. Many 

 backboneless animals of higher degree on different lines of 



