786 CONCLUSION xxxii. 18 



During the present study we have not been able to show rigorously 

 that all evolution follows such principles, but the data are not incon- 

 sistent with this. It may be that vertebrates have proceeded farther 

 along the path suggested than any other animals. The birds and 

 mammals probably turn over energy faster than other vertebrates. The 

 point that must not be overlooked is that they do this in order to 

 provide the means by which they can remain constant, in spite of 

 fluctuations in the external conditions of an environment that is 

 different in composition from the living system. 



We cannot then at present discern with certainty the principles that 

 determine the change of animal form, but we can see something of the 

 influences that have modified the original vertebrate type to produce 

 the great variety of creatures that has existed and remains today. We 

 cannot give tables of numbers to show how the variables have operated 

 to keep Ceratodus nearly constant for 200 million years while related 

 descendants have gone on to produce the whole variety of tetrapod life. 

 But we can suggest that it is worth while pursuing the study as a means 

 of building a truly general science of zoology. We begin to see signs of 

 the principles according to which the vast populations of animals 

 interact with each other, with the plants, and with the inorganic world. 

 The changes that these interactions produce seem to be, if not con- 

 stantly in one direction, at least often such as to allow the appearance 

 of more varied and complicated forms of life, ever more able to maintain 

 themselves constant and apart from the environment, and hence to 

 exist in a wider range of conditions. The effect of this evolutionary 

 change has probably been to increase the amount of material organized 

 into living things, while the total energy flux through the system has 

 also been enlarged. 



This finding is not a mere rehabilitation of the complacent anthro- 

 pocentric prejudices of the nineteenth century. The conclusion that 

 there is a sense in which the mammals and man are among the highest 

 animals should be based on objective analysis of the behaviour of the 

 populations of vertebrates and the flow of information and energy 

 through them. However, it has been repeatedly emphasized that we 

 still have a long way to go before we have the knowledge necessary to 

 understand these very slow changes. It has only been possible in this 

 volume to suggest certain principles that may one day make possible 

 a satisfactory systematic study of the life of vertebrates. 



