212 PALEONTOLOGY. 



Should these facts receive confirmation by subsequent in- 

 vestigation, we shall be enabled to deduce some interesting 

 consequences on the state of the globe during the different 

 epochs of its history. 



The four preceding laws seem to be established on sound 

 premises ; but that which remains to be enunciated must be 

 received with considerable reservation, as it involves the 

 right interpretation of physiological questions which are far 

 from being clearly understood. 



FIFTH LAW. The faimas of the most ancient strata are 

 composed of animals of a more simple organisation, and their 

 degree of perfection increases in proportion as we approach 

 epochs more recent. 



In the brief space allotted to this chapter, it is impossible 

 to discuss this law on its merits ; and therefore we must 

 refer to Pictet's admirable work for an exposition of this 

 branch of the subject. It may be observed, however, that 

 if the law is limited to the grouping of species, or the 

 numeric proportion between the vertebrate and invertebrate 

 animals, then it is true that the absence of the higher 

 organisms from the ancient rocks supports the correctness 

 of the proposition. But if this fact is mixed up with theo- 

 retical ideas on the progressive development of species, 

 through long periods of time, then it becomes a question of 

 hypothesis, and ceases to be one of sound deduction. 



The animal kingdom is divided into four great groups : 



1. EADIATA. 



2. MOLLTJSCA. 



3 . AETICULATA. 



4. YEBTEBEATA. 



This division is based on the structure, development, and 

 mode of arrangement of the nervous system, to which the 

 dermal* and osseous skeletons may be considered as the 

 appanage. It is, therefore, a strictly natural grouping of 

 the primary forms of animals. 



We shall pass in review each of the classes to which 

 fossils belong, commencing with the lowest and ascending 

 to the highest, 



* (Gr. Sepfta, skin) belonging to the skin. 



