784 CONCLUSION xxxii. 16- 



that the influence of pathogens also puts a premium on variation. The 

 disease organism, being small, can usually evolve more rapidly than 

 the host, and it is therefore an advantage for a member of the host 

 population to be different from the majority, to which the pathogen is 

 adapted. 



During the present study there has been little progress towards 

 demonstrating that these are the factors influencing evolution, though 

 there are clear signs that some of them are at work, for instance, 

 geographical isolation. They are treated in detail by such works as 

 those of Huxley (1942) and Simpson (1953) and there properly dis- 

 cussed. These questions are raised here only as a reminder that they 

 will have to be further considered if any satisfactory general evolu- 

 tionary theory is to emerge and to be applicable to the vertebrates. 



17. The direction of evolutionary change 



Since we cannot closely specify the factors influencing evolution 

 we can hardly expect to go far towards the solution of the still more 

 difficult problem of the direction of evolutionary change. The facts 

 of importance that have emerged from the evidence about vertebrates 

 are (1) That populations tend to be replaced by others of different 

 form, often themselves descended from the first. (2) That the later 

 populations often have more complicated organizations than the earlier 

 ones; we can hardly be said to have established this last clearly as a 

 rule in vertebrates, but it has repeatedly been suggested that the 

 evidence supports some such thesis. (3) Some later populations invade 

 habitats not previously occupied by vertebrates (e.g. the land). 



Facts of this sort have led us repeatedly to the suspicion that the 

 later types are often in some way better able to carry on the self- 

 maintenance of life than their predecessors, and that vertebrate life is 

 continually invading new situations. With this goes the suspicion that 

 the total biomass of vertebrate life (perhaps of all life) has been increas- 

 ing and the energy flux becoming more rapid, though we have no exact 

 estimates to verify this. 



For organizing our knowledge about the evolution of vertebrates or 

 other animals the above three facts about the progress or direction of 

 change are of great value. The invasion of new habitats is of particular 

 interest, because it is usually made possible by the increase of com- 

 plexity, this provides a system of elaborate adjustments that maintains 

 the internal conditions nearly constant in face of fluctuations in the 

 environment. This internal constancy or homeostasis is, of course, 



