INTRODUCTION 517 



differences of method in making the best use of them. The differences 

 are of degree rather than of kind. 



Principles 



The fundamental principle which underlies all faunal correlation is 

 evolutionary change. Every species, every race, is changing more or less 

 slowly, giving rise to new species, branching out into various forms or 

 becoming extinct and succeeded by other races which have been similarly 

 changing. These changes do not proceed uniformly over all parts of the 

 world, but new types appear in one region, spread more or less widely 

 over the earth's surface, and are in their turn succeeded by others. Prim- 

 itive types may linger in remote and isolated regions long after they have 

 been displaced elsewhere by more progressive stages. They may survive 

 through the adoption of some special mode of life that withdraws them 

 from competition with the more progressive species, or they may acquire 

 some special mode of protection which enables them to survive. -In none 

 of these cases do they survive unaltered; always they are changed more 

 or less in adaptation to their special habitat," but are less advanced than 

 their more progressive contemporaries. 



The rate of change varies widely in different races. The- higher and 

 more complex animals are more sensitive to external conditions; their 

 changes in structure are far more rapid and obvious. This is especially 

 true of the higher vertebrates. 



The evolutionary changes are essentially an adaptation to a continually 

 changing environment. The inherited characters of the race have been 

 gradually assumed and impressed on it by the influence of the successive 

 environments through which its ancestors have passed. The acquired 

 characters are assumed in response to its adaptive needs. Similar adapta- 

 tions imposed on two different races will result in the superposition of a 

 superficial resemblance due to the identity of environment on a more 

 fundamental difference due to different inheritance. 



It is in these matters that the difficulties of correlation lie. We have 

 to distinguish if we are to obtain true correlations between persistently 

 primitive survivals and the similar but more ancient forms from which 

 they are derived. We have to distinguish between resemblances in the 

 faunas due to similar environment and those conditioned by similar age. 

 The sure recognition of these distinctions depends on the completeness 

 of our evidence. If the fossil remains of the animal enabled us to know 

 its structure completely and in detail, it might be difficult, but would 

 always be possible, to- distinguish'; but the -fossil remains afford only in- 

 complete and partial evidence'. Internal structure affords better evidence 



