THE RISE OF THE MAMMALIA 135 



It may have struck the reader that in our short 

 and necessarily incomplete survey of the animal 

 kingdom, we have been able to speak with far more 

 confidence and certainty as to the course evolution 

 has taken, when dealing with the Vertebrata than 

 with the Invertebrata. This is to be expected on the 

 theory of evolution, as the Vertebrata are the more 

 modern products and there has been less time for 

 extermination to wipe out the records of the con- 

 necting links. Within the limits of the Invertebrate 

 phyla we are able to convince ourselves that a simi- 

 larity of plan runs through the various components, 

 but the incompleteness of the geological record makes 

 the task of deciding as to which component is more or 

 less primitive exceedingly difficult. We have seen 

 that the phyla themselves are of immense antiquity ; 

 that their origins go back probably far beyond the 

 limits of geological time as reckoned by stratified, 

 fossil-bearing rocks, so that the attempt to determine 

 the relationships of these phyla must remain a matter 

 of vague hypothesis or cautious scepticism. This 

 limitation, however, need not shake our faith in the 

 truth of the doctrine of evolution or of descent with 

 modification, as it is inherent in the nature of the 

 evolutionary process, being the outcome of the anti- 

 quity of life, of the prodigality of form in which life 

 has been manifested, and of the vast scale on which 

 extinction has taken place. 



