VI. 1 SPECIES AND TIME. I49 



with more and more complete segmentation and ossifica- 

 tion of the backbone, which, in the earliest forms, w\is (as 

 it is in the lowest fishes now) a soft, continuous rod or 



Tnii.oniTE. 



notocliord. Now, however, it is considered probable that 

 the soft backboned Labyrinthodon Archegosaurus was an 

 immature or larval form,^ while Labyrinthodon ts, with com- 

 plet(^ly developed vertcbrrc, have been found to exist among 

 the very earliest forms yet discovered. The same may be 

 said regarding the eyes of the trilobites, some of the oldest 

 forms having been found as w^ell furnished in that respect 

 as the very last of the group which has left its remains ac- 

 cessible to observation. 



Such instances, however, as well as the way in which 

 marked and special forms (as the Pterodactyls, etc., before 

 referred to) appear at once in and similarly disappear from 

 the geological record, are of course explicable on the Dar- 

 winian tiieory, provided a sufficiently enormous amount of 

 past time be allowed. Tlie alleged extreme, and probably 

 great, imperfection of that record may indeed be pleaded 

 in excuse. But it is an excuse." Nor is it possible to deny 



' As a tadpole is the larval form of a frog. 



^ Afl Prof. TTnxloy, with his characteristic candor, fully admitted in 

 his lecture on the Dinosauria before referred to. 



