54 INTRODUCTION. 



reasonably be appealed to as evidence on this question. The 

 stratified series is long enough to render it certain that it con- 

 tains traces of the first appearance of, at any rate, the higher 

 classes of these, though we doubtless are ignorant of the abso- 

 lute moment at which each appeared. If, therefore, it can be 

 shown that there has been a progression as far as this sub- 

 kingdom is concerned, then there would, by analogy, be the 

 greatest probability 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 position, 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 ex- 

 ample 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 no structures by 

 which we can reasonably expect to find them recorded 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 gen- 

 erally regarded as inferior to the Ganoids and Placoids in zoo- 

 logical position. This, however, is a matter of opinion, and 

 reasons are not wanting for regarding them as the highest of 

 their class. 



It only remains to add that nothing further is contended for 

 here than the general fact of there having been a progression 

 of morphological types, the lowest presenting themselves first, 

 the highest being the last to appear upon the scene. It is by no 



