EVOLUTION AND MANKIND. 2gi 



rise to them. Their occurrence in time can be ascertained by 

 consulting a i)alaeozoo!ogical table of strata. The exact origin of 

 vertebrates is not agreed upon with unanimity ; there are several 

 dififerent views thereon. One viev,-, put forward by a physio- 

 logist, the late Dr. (raskell, was that vertebrates might have arisen 

 from certain .\rthropod-like ancestors, which learned to live and 

 swim on their backs. Forms somewhat resembling the supposed 

 Arthrojx>d ancestor are the present day Limulus or king crabs, 

 found off the muddy or sandy shores of the Atlantic coast of the 

 United States of America and the China seas. Limulus is the 

 sole surviving representative oi its race. The youth form or 

 larval stage of Limulus presents a striking superficial resemblance 

 to some of the extinct Trilobites. 



The fossil Eurypterida, allies of Limulus and of present-day 

 scorpions, were large forms found from the Cambrian to the 

 Carboniferous strata. An example of the Eurypterids is Ptery- 

 gotus osiliensis, a sort of large sea scorpion. Such forms are also 

 interesting in that they bear a superficial resemblance to some of 

 their contemporaries, the Ostracoderms, a remarkable group of 

 fishes with dermal armour found in the Upper Silurian and Devon- 

 ian rocks. The Ostracoderms are the oldest known vertebrates. 

 The resemblance between the Eurypterids and the Ostracoderms 

 may be merely superficial and due to convergence, owing to 

 similar conditions of life. 



The Ostracoderms, owing to their absence of jaws, are placed 

 with the lampreys and hagfishes in the Agnatha. An interesting 

 small fossil member of the group is Palaeospoiidylus. a small 

 tadpole-like creature about one or two inches long, a lamprey of 

 the Devonian age. 



The most primitive group oi true jaw-bearing or cartilagin- 

 oiis fishes is the Elasmobranchii, which date from Silurian times, 

 and which include sharks, dogfishes, skates and rays. The bony 

 fishes or Teleostei. including the great majority of living fishes, 

 are of later origin. 



The interesting and remarkable lung fishes or mud fishes, 

 called the Dipnoi, are feebly represented at the present day by the 

 Australian Ccratodus {Neoceratodus) , the African Protopterus 

 and the South American Lepidosircn. These fish, the survivors 

 of an archaic group, indicate how the transition from an aquatic 

 •to a terrestrial mode of life was first rendered possible to verte- 

 brate animals, by the conversion of the swim bladder into a pair 

 of lungs. The Dipnoi arose and attained their maximum develop- 

 ment during the Devonian epoch. Unfortunately, here an illus- 

 tration is afforded of the occasional imperfection of the geological, 

 record, as the rocks yield no evidence as to how the paired fins of 

 fishes gave rise to the pentadactyl or 5-digitate limbs of the semi- 

 terrestrial Amphibia. 



The earliest known Amphibia were the Stegocephalia. They 

 ranged from the Carboniferous to the Upper Trias. As their 

 name implies, they had strongly developed bony armour, which 

 covered the head and formed the roof of the skull, as occurred 



