INTRODUCTION 5 



the history complete. If, in human history, we had only the 

 records for one century in China, for another in England, and for 

 yet another in South America, how imperfect indeed would be our 

 knowledge of human progress. Animals and plants are never quite 

 alike in remote regions, and they never have been. The living 

 reptiles of North and South America are today almost entirely 

 different, and, were their fossil remains to be discovered a million 

 years hence, it would be very difficult to decide that they had once 

 lived contemporaneously; difficult, though perhaps not impossible, 

 since some are so nearly alike that their relationships or possible 

 identity would probably be established after long search. This 

 will serve to make clear how very difficult it is, for the most part, 

 to correlate exactly the geological formations in remote regions 

 of the earth, or even sometimes in adjacent regions where the fossils 

 are scanty, or the conditions under which the animals had lived 

 were very different. 



There are long periods of time, millions of years at a stretch 

 perhaps, throughout which our knowledge amounts to little or 

 nothing concerning many land reptiles which we are sure must 

 have existed abundantly. No better example of our oftentimes 

 scanty knowledge can be cited than the following. Until within 

 the past fifteen years it was thought that true land lizards, of which 

 there are about eighteen hundred species now living, dated back 

 in their history no farther than about the close of the great Second- 

 ary Period, or the Age of Reptiles. But a single skull of a true 

 land lizard has been discovered in the Triassic deposits of South 

 Africa, a skull of a form so nearly like that of the modern iguana 

 of America that its discoverer, Dr. Broom, has called it Paliguana. 

 The lizards must have been in existence, probably many thousand 

 species of them, during all the great interval of time between 

 the Middle Triassic and the close of the Cretaceous, since it 

 is a law which can have no exception, that a type of life once 

 extinct never reappears. The "ancient iguanas" of the Trias 

 must have been the forbears of many, if not all, of the lizards of 

 later times, though nothing is known of their descendants through 

 a period of time which can be measured only by millions of 

 years. 



