THE ANCESTRY OF THE CHORDATA. 571 
Though it is likely that many of the suggestions here made 
may be shown hereafter to be wrong, still it has seemed well, 
on the whole, to analyse the facts as they stood, and to endea- 
vour to reconstruct the past stages, whose existence is indi- 
cated by the lacune in the sequence of these facts, avoiding 
as far as possible a reliance upon phylogenetic changes of 
whose occurrence we have no evidence. 
The foregoing views are, perhaps, more clearly expressed in 
the following table, which is not meant so much as a genealo- 
gical tree as to serve as an exhibition of the logical relation 
of the various forms, showing their points of divergence. 
Protognathostomata 
(of Balfour). 
Cyclostomata. 
Cephalochorda. 
Enteropneusta. 
ie Tunicata. 
P (| 
Form with one gill-slit. 
