152 THE GENESIS OF SPECIES. [CHAP. 



Pliocene period, lias no support from observation or 

 analogy." 



Not only, however, do we fail to find any traces of the in- 

 cipient stages of numerous very peculiar groups of animals, 

 but it is undeniable that there have been instances which 

 appeared at first to indicate a gradual transition, and yet 

 these instances have been shown by further investigation 

 and discovery not truly to indicate anything of the kind. 

 Thus at one time the remains of Labyrinthodonts which 

 up till then had been discovered, seemed to justify the 

 opinion that, as time went on, forms had successively 

 appeared with more and more complete segmentation and 



TRILOBITE. 



ossification of the backbone, which in the earliest forms 

 was (as it is in the lowest fishes now) a soft continuous rod 

 or notochord. Now, however, it is considered probable that 

 the soft back-boned Labyrinthodont (Archegosaurus) was 

 an immature or larval form, 1 while Labyrinthodonts with 

 completely developed vertebrae have been found to exist 

 amongst the very earliest forms yet discovered. The same 

 may be said regarding the eyes of the trilobites, some of 



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



