26 THE AGE OF THE EARTH 



Before proceeding further, I wish to lay emphasis on 

 the immense evolutionary history which must have been 

 passed through before the ancestor of one of the higher 

 of these nine Phyla came into being. Let us consider 

 one or two examples, since the establishment of this 

 position is of the utmost importance for our argument. 

 First, consider the past history of the Vertebrata, of 

 the common ancestor of our Balanoglossus, Tunicates, 

 Amphioxus, Lampreys, Fishes, Dipnoi, Amphibia, Rep- 

 tiles, Birds, and Mammals. Although zoologists differ 

 very widely in their opinions as to the affinities of this 

 ancestral form, they all agree in maintaining that it did 

 not arise direct from the Nematophora in the lower sub- 

 grade of Metazoa, but that it was the product of a long 

 history within the Coelomate sub-grade. The question 

 as to which of the other Coelomate Phyla it was associated 

 with will form the subject of one of our discussions at 

 this meeting, and I will, therefore, say no more upon this 

 period of its evolution, except to point out that the very 

 question itself, ' the ancestry of Vertebrates,' only means 

 a relatively small part of the evolutionary history of the 

 Vertebrate ancestor within the Coelomate group. For 

 when we have decided the question of the other Coelo- 

 mate Phylum or Phyla to which the Vertebrate ancestor 

 belonged, there remains of course the history of that 

 Phylum or those Phyla earlier than the point at which 

 the Vertebrates diverged, right back to the origin of the 

 Coelomata ; while, beyond and below, the wide gulf 

 between this and the Coelentera had to be crossed, and 

 then, probably after a long history as a Coelenterate, the 

 widest and most significant of all the morphological 

 intervals that between the lowest Metazoon and the 

 highest Protozoon was traversed. But this was by no 

 means all. There remains the history within the higher 

 Protozoan sub-grade, in the interval from this to the 

 lower, and within the lower sub-grade itself, until we 

 finally retrace our steps to the lowest and simplest forms. 

 It is impossible to suppose that all this history of change 

 can have been otherwise than immensely prolonged ; for 

 it will be shown below that all the available evidence 



