174 THE DARWINIAN THEORY 



(2) Development. — Here we obtain evidence from 

 the Recapitulation theory, or the tendency of animals 

 to repeat their past history in actual development. 



(3) Fossils. — These afford the most valuable of 

 all evidence, because it is the most direct and con- 

 vincing, although at the same time the most frag- 

 mentary and incomplete. 



Vertebrates form a good group for our purpose, 

 inasmuch as in them evidence of all three kinds is 

 available. The main characteristics of a typical 

 Vertebrate are the tubular nervous system, forming 

 the brain and spinal cord ; the notochord, forming 

 the main skeleton or backbone, and situated between 

 the nervous system and the alimentary canal ; the 

 myelonic eye, or eye developed from the brain. 



Classification of Vertebrates. 



I. Craniota. — In these the skull and brain are 

 present, and limbs nearly always so, or when absent 

 have been clearly lost. The heart, liver, and other 

 organs are well developed. 



0. Pisces, or fish, are aquatic Vertebrates possess- 

 ing gills, and provided with fins instead of limbs. 

 They are rarely able to leave the water. 



b. Amphibia, such as frogs, newts, and toads, 

 are fresh-water or terrestrial. In early life they are 

 aquatic, breathing by gills. Later on in life they 

 may lose their gills and take to land life. 



c . Reptilia. — These never have gills. They 

 possess fore and hind limbs, furnished with fingers 

 and toes, except in cases where they have been lost, 



