6o VESTIGES OF THE 



the Palaeozoic period [placoicls and ganoids], the caudal 

 fin is heterocercal, being formed of two unequal branches, 

 the upper one expanded immediately from the vertebral 

 column, while the lower one is given off at a point some 

 distance from the extremity." * iSTow it is a remarkable 

 fact, that this one-sided tail is a peculiarity in the more 

 perfect fishes (as the salmon) at a certain stage in their 

 embryonic history ; as is also the inferior position of the 

 mouth, peculiar to the early fishes. More than this — in 

 the earlier periods of embryonic life, there is no vertebral 

 column. This organ is represented in embryos by a 

 gelatinous cord, called the dorsal cord, which in ma- 

 turity disappears as the vertebrae are formed upon it. 

 M. Agassiz has satisfied himself that this was the nature 

 of the organisation of the placoid and ganoid fishes, as it 

 is that of the sturgeon of the present seas. It is not 

 premature to remark how broadly these facts seem to 

 hint at a parity of law afiecting the progress of general 

 creation, and the progress of an individual foetus of one 

 of the more perfect animals. 



It is equally ascertained of the types of being preva- 

 lent in the old red, as of those of the preceding system, 

 that they are uniform in the corresponding strata of 

 distant parts of the earth; for instance, Kussia and 

 North America. 



In the old red sandstone, the marine plants, of which 

 faint traces are observable in the Silurians, continue to 

 appear. It would seem as if less change took place in 

 the vegetation than in the animals of those early seas ; 

 and for this, as Mr. Miller has remarked, it is easy to- 

 imagine reasons. For example, an infusion of lime into 

 the sea would destroy animal life, but be favourable to 



* An steel's Geology, i, 185. 



