92 INTRODUCTION. 



certain that it contains traces of the first appearance of, 

 at any rate, the higher classes of these, though we doubt- 

 less are ignorant of the absolute moment at which each 

 appeared. If, therefore, it can be shown that there has 

 been a progression as far as this sub - kingdom is con- 

 cerned, then there would, by analogy, be the greatest pro- 

 bability that a similar progression has taken place in all the 

 sub-kingdoms. 



So far as our present knowledge goes, it would appear 

 that there is such a progression in the Vertebrate sub- 

 kingdom. The classes of Vertebrates make their appearance, 

 on the whole, in the order indicated by their zoological posi- 

 tion, the lowest first and the highest last. Not only does 

 this hold good for the classes of the Vertebrates, but the 

 same general statement may be made as to the orders of 

 each class. Where apparent exceptions occur, a reasonable 

 explanation can be given, or our knowledge can be shown to 

 be defective. Space will not allow a discussion of this 

 question, but a single example may be taken. So far as we 

 know at present, the earliest remains of Vertebrate animals 

 are those of Fishes the lowest class of the sub-kingdom 

 and these appear in the Upper Silurian rocks for the first 

 time. Granting the probability that Fishes may some day 

 be found in the Lower Silurian rocks, or even in Cambrian 

 deposits, there still seems no likelihood that they will be 

 deprived by any future discoveries of their position as being 

 the earliest of their sub-kingdom. The oldest remains of 

 Fishes, however, are by no means those which would be 

 expected, but belong to two of the higher orders of the class. 

 This seeming anomaly, however, disappears when we consider 

 that the two lowest orders of Fishes possess almost no struc- 

 tures by which we can reasonably expect to find them re- 

 corded in a fossil state. They may therefore have been in 

 existence long before the Ganoids and Placoids of the Upper 

 Silurian rocks, and we have no right to assume that they 

 were not. As to the remaining great order of Fishes (the 

 Teleostean Fishes), it is certain that their appearance was 

 much later, and they are generally regarded as inferior to 

 the Ganoids and Placoids in zoological position. This, how- 



