12 THE GEOLOGICAL EVIDENCES 



of the known cycle of geological history, or com- 

 pletely through what is known as the Paleozoic 

 period of time. In the Triassic period we have 

 the first evidence of the existence of the highest 

 animals, the mammals, and in the period following 

 this, the Jurassic, of birds, an apparent contradic- 

 tion to the order of appearance. 



Let us here enter somewhat more closely into an 

 examination of the order of appearance that has 

 been outlined, and see what it signifies. I believe 

 we shall find in its analysis both kinds of 

 evolutionary proof that we are in search of. But 

 in order to do this we must satisfy ourselves as 

 to the relationships to one another of the differ- 

 ent animal groups whose histories we have 

 followed. What is a fish, what is an amphibian, 

 and what is a reptile, and what relationship 

 do these three groups bear to one another ? 

 I can in this place only briefly indicate the 

 essential anatomical features of these groups. 

 Beyond having the characters belonging to the 

 Vertebrata in general, fishes may be described as 



