384 THE TRANSITION TO LAND 



Prehistoric Reptiles. At one time, reptiles were the dominant animals 

 of the earth. There were the huge land dinosaurs, that were much larger 

 than any land animal of today, as well as many smaller land reptiles that 

 dominated life on the land. There were flying reptiles that were abun- 

 dant in the air and tremendous swimming reptiles that were extremely 

 abundant in the waters of the earth. Through studies of prehistoric and 

 modern reptiles, we have been able to identify 125 families in this class. 

 Of this great number, however, only eighteen are found alive on the earth 

 today. These show particular adaptations that enabled them to survive 

 while their many relatives became extinct. 



Photo by Winchester 



Fig. 26.6. A snapping turtle. This turtle looks rather sluggish but it has a reputation 

 for quick action as it snaps for food or a finger that might be held too close. 



The decline of the reptiles was accompanied by a rise of the mammals. 

 There are a number of theories to account for this change of emphasis 

 in the animal world but one of the most plausible is based on climatic 

 changes which are known to have occurred in the past. It is known that 

 the climate of the United States, for instance, has varied from tropical 

 to frigid during past geological eras. Reptiles are cold-blooded animals 

 and may be quite active in warm weather, but when the temperature 

 drops their muscles become sluggish in their movements and they become 

 entirely inactive when it gets down around 45 degrees or below. A 

 rattlesnake placed in a refrigerator becomes quite harmless until he gets 

 warmed up again. The opposite is true of mammals ; they may feel slug- 

 gish in very warm weather, but as the weather becomes brisk they be- 

 come more and more active. During tropical climatic periods the rep- 

 tiles could dominate the mammals, but as the weather got cool small 

 aggressive mammals could easily overcome the larger and stronger rep- 

 tiles and exterminate many of them. 



Turtles. These reptiles have traded activity for protection. They 

 are slow and awkward, but so well protected by their outer "shell" that 



