492 



MESOZOIO ERA— AGE OF REPTILES. 



and mammals. We give below figures taken from Marsh, showing the 

 relative size of the brain in living and Cretaceous birds. 



Mammals. — It is a most remarkable fact that although Marsupial 

 mammals have been found in the Jurassic, and probably existed in 



considerable numbers then, yet, ex- 

 cept in the Laramie which may be 

 regarded as a transition to the Ter- 

 tiary, not one has been found in 

 the Cretaceous. We know they 

 existed at that time, for they are 

 found in the Laramie of America 

 and in the Tertiary of both Europe 

 and America, and still exist in Aus- 

 tralia and elsewhere ; and it is a 

 well-established law in Paleontol- 

 ogy that if a type becomes extinct 

 it never reappears: Evolution never 

 goes backward : Nature never re- 

 peats herself. It is probable, there- 

 fore, that during the Cretaceous the 

 Marsupials which doubtless existed 

 had been driven to some other por- 

 tion of the earth, where we shall yet 

 find their remains when our knowl- 

 edge of the geology of the globe is 

 more complete ; and in them we 

 shall also probably find the transi- 

 tions to, or earliest progenitors of, 

 the True Mammals of the Tertiary. 



Fig. 826. 



Fig. 827. 



Figs. 826, 827.-826. Outline of the skull and 

 brain - cavity of Ichthyornis victor (after 

 Marsh), seen from above. Five sixths nat- 

 ural size. 827. Outline of the skull and 

 brain-cavity of Sterna cantiaca (after Gme- 

 lin), same view. Natural size, ol, olfactory 

 lobes ; e, cerebral hemispheres ; op, optic 

 lobes; cb, cerebellum. 



Continuity of the Chalk. 



It is probable that the deep At- 

 lantic Ocean bottom, where chalk is now forming, is continuous with 

 the chalk of England and Central Europe. In other words, in Creta- 

 ceous times a deep sea ran from the mid- Atlantic far into what is now 

 Central Europe, and in the whole of this deep sea chalk was then 

 formed. At the end of the Cretaceous period the eastern part was 

 raised and formed a portion of Europe, while the rest remained as 

 deep-sea bottom, and continued to make chalk until now. Thus there 

 is no doubt that in the deep Atlantic, off the coast of Europe, there has 

 been an unbroken continuity of chalk-making from the Cretaceous 

 times until now. But we have seen (p. 478) that many of the living 

 deep-sea species are identical with, and nearly all extremely similar to, 

 those found in the chalk of Cretaceous times. Thus there has been 



