THE EVOLUTION OF ORGANIC TYPES IN TIME. 103 



pointed out, that it is very doubtful if we are as yet acquainted with 

 the absolute time of the first appearance upon the globe of even one 

 of the sub-kingdoms. Future discoveries, therefore, are almost cer- 

 tain to push back still further into the remote vistas of the past the 

 point of time at which each morphological type first made its appear- 

 ance upon the globe. Nevertheless, there is little likelihood that the 

 relative times of appearance of the great groups, as compared with 

 one another, will be affected by any discoveries which we have yet to 

 make. Moreover, as regards the Invertebrate sub-kingdoms, we are, 

 perhaps, never likely to find any reliable evidence which would 

 enable us to fix with precision their relative order of appearance. 

 x\ll of these sub-kingdoms appear in the Cambrian deposits, and the 

 utmost that we can be certain of is that they had been in existence 

 in times long anterior to the Cambrian. Owing, however, to the 

 very general metamorphism which has affected the pre-Cambrian 

 sediments, we cannot hope to ever obtain more than the most scanty 

 evidence, if any, as to the actual origin of the earliest types of 

 life. With regard to the Vertebrate animals, on the other hand, the 

 series of the fossiliferous rocks is long enough to render it certain 

 that within its limits we ought to find traces of the first appearance 

 of, at any rate, the higher classes of these, though we are doubtless 

 likely to remain ignorant of the precise stage in the series at which 

 each first made its appearance. If, therefore, it can be shown that 

 there has been a progression so far as this sub-kingdom is concerned, 

 then there would, by analogy, be the greatest probability that a 

 similar progression has taken place as regards all the sub-kingdoms. 

 So far as our present knowledge goes, it would appear certain 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. Where apparent exceptions occur, a reasonable explan- 

 ation can be given, or our knowledge can be shown to be defective. 

 Thus, the Fishes constitute the lowest group of Vertebrates, and in 

 accordance with this they are the first to make their appearance (in 

 the Silurian period). The Amphibians stand next to the Fishes in 

 zoological rank, and they are the next group of Vertebrates to make 

 their appearance, their earliest known representatives being found 

 towards the close of the Palaeozoic period (in the Carboniferous 

 system). A little later than the Amphibia (in the Permian period) 

 appear the Reptiles, which constitute the next highest class of Ver- 

 tebrates. The class of Birds, the next in the series, possesses a 

 palaeontological record so fragmentary that we may leave it out of 

 account in this connection ; but the last and highest group of Verte- 

 brates, that of the Mammals, appears for the first time towards the 

 close of the Triassic period. It need only be added, that the gene- 



