THE APPENDICULATB PHYLUM 57 



results which a single animal, Peripatus, has yielded, 

 by indicating how the Arthropod type of body-cavity 

 and vascular system has been derived from the Anne- 

 lid. But Peripatus is of fundamental importance 

 only in this respect. In dealing with the various 

 classes of Arthropods, we found that Peripatus throws 

 no light on the derivation of these classes from some 

 common Arthropodan ancestor. Again, in dealing 

 with these several classes we found that they existed 

 in a fully differentiated condition in the oldest fossil- 

 bearing rocks known to us, and that even existing 

 families, if not genera, were present at the most 

 remote geological periods and have suffered little or 

 no important changes through all the incalculable ages 

 that have supervened from that time to this. 



We seem to be confronted by two apparently 

 opposing bodies of fact ; on the one hand, the immense 

 antiquity and stability of living forms, on the other 

 the evidence of a vast process of change and ex- 

 tinction. But at least we may have the satisfaction 

 of knowing the reason of our perplexity, namely that 

 we are trying to disentangle the plot of a drama of 

 which we are permitted to be the spectators of only 

 the closing scenes. 



