36 Br Gaskell, The Origin of Vertebrates. [Nov. 25, 



and it seems to me much more probable that the yolk is there 

 because that part is last formed, than that it is last formed because 

 the yolk is there. 



The palceontological evidence. 



It is clear from what has been said that the evidence of 

 palaeontology ought to show, firstly, that the Vertebrates appeared 

 when the waters of the ocean were peopled with the forefathers of 

 the Crustacea and Arachnida, and secondly, the earliest fish-like 

 forms ought to be characterised by the presence of a large cephalic 

 part to which is attached an insignificant body and tail. Such 

 was clearly the case, for the earliest fish-like forms appear in the 

 midst of and succeeding to the great era of strange proto- 

 crustacean animals, when the sea swarmed with Trilobites, 

 Eurypterus, Slimonia, Limulus, Pterygotus, Ceratioceras and a 

 number of other semi-Crustacean and semi- Arachnid creatures ; 

 and when we examine these ancient fishes we find such forms as 

 Pteraspis, Ptericthys, Astrolepis, Bothriolepis, Cephalaspis, all 

 characterised by the enormous disproportion between the extent of 

 the head region and that of the body. Such forms would have but 

 small power of locomotion, and further evolution consisted in 

 gaining greater rapidity and freedom of movement by the elonga- 

 tion of the abdominal and tail regions, with the result that the 

 head region became less and less prominent, until finally the 

 ordinary fish-like form was evolved, in which the head and gills 

 represent the original head and branchial chamber, and the 

 flexible body with its lateral line nerve and intestine innervated 

 by the vagus represent the original small tail-like body of such a 

 form as Ptericthys. 



Nay more, the very form of Ptericthys and the nature of its 

 two large oar-like appendages, which according to Traquair are 

 hollow like the legs of insects, irresistibly remind one of a form 

 like Eurypterus in which the remaining appendages had shrunk to 

 tentacles as in Ammoccetes, but the large oar-like appendage still 

 remained, coming out between the lower and upper lips and 

 assisting locomotion. The Ammoccetes-like forms which existed 

 between the time of Eurypterus and the time of Ptericthys not 

 having developed bony plates have not yet been found, although 

 the recent find of Traquair of Palaeospondylus Gunni is exceedingly 

 interesting in this respect. The evidence of palaeontology as far 

 as it goes confirms absolutely the evidence of anatomy, physiology, 

 phylogeny and embryology and assists in forming a perfectly consis- 

 tent and harmonious account of the origin of Vertebrates, the whole 

 evidence showing how nature made a great mistake, how excellently 

 she rectified it, and thereby formed the new and mighty kingdom 

 of the Vertebrata. 



