MUD-FISHES. 115 



Osseous Fishes. The Primitive Fishes (Selachii) form the 

 most ancient and original group of Fishes. From these, 

 in one direction, all other Fishes have developed ; the 

 Mucous Fishes first, which, at a much later period (in the 

 Jurassic, or the Chalk Period), gave rise to the Osseous 

 Fishes. In another direct'on, the Primitive Fishes gave 

 rise to the parent-forms of the higher Vertebrates, directly 

 to the Dipneusta, and thus to Amphibians. Regarding the 

 Selachii as forming the eleventh stage in our pedigree, these 

 would be followed by the Dipneusta group as the twelfth 

 stage, and by the Amphibian group as the thirteenth stage. 

 The advance effected in the development of the Mud- 

 fishes (Dipneusta) from the Primitive Fishes is of great mo- 

 ment, and is connected with a very noticeable change, which 

 took place in the beginning of the Palaeozoic, or Primary 

 Period in organic life as a whole. For the very numerous 

 fossil remains of plants and animals which are now known to 

 belong to the first three epochs of the history of the earth 

 to the Laurentian, the Cambrian, and the Silurian Periods, 

 are exclusively those of aquatic plants and animals. From 

 this paloeontological fact, taken in connection with certain 

 weighty geological and biological considerations, we may 

 infer, with tolerable certainty, that at that time no land 

 animals yet existed. During the whole of the enormous 

 Archizoic Period during many millions of years the living 

 population of our globe were all water-dwellers : a very 

 remarkable fact, when it is remembered that this period 

 embraces the larger half of the entire organic history of the 

 earth. The lower animal tribes are even now exclusively, 

 or with very few exceptions, aquatic. But during the 

 Archizoic, or Primordial Epoch, the higher animal tribes 



